I would consider preboarding- note you should ask at the gate- they may not announce preboarding for people with disabilities- they should if they announce preboarding for children, but sometimes they don't. This would give you an extra minute or two to settle in.
You didn't specify whether your new seats were AB, BC, DE, or EF- if either AB or EF, there's a window next to one of the seats. The airline doesn't care if you or your traveling companion trade seats. This would enable your son to have a wall (and maybe a window) next to him, and might help create some safe space. If the pair of seats are BC or DE, then the single is a window (or if the pair is split, say AC or DF, there's a middle seat. If your husband has a A, C, E,or F seat, chances are the person in the middle of you and your son would be happy to trade to an aisle or window.
I'd also be sure to ask at the gate whether the flight is full- if not, it will be easier to shuffle seats.
There are some rules about seat assignments (see below). I would guess that you or your husband would qualify as an attendant. You may need to enquire further as to whether a second attendant is permissible or would qualify as an accommodation.
The airline MUST have a Complaint Resolution Officer on duty at the airport at all times to address complaints associated with accommodations.
-steve
From 14 CFR Part 382
Nondiscrimination on the Basis of Disability in Air Travel
(Federal rules on air travel)
Subpart F Seating Accommodations
§382.81 For which passengers must carriers make seating accommodations?
As a carrier, you must provide the following seating accommodations to the following passengers on request, if the passenger self-identifies to you as having a disability specified in this section and the type of seating accommodation in question exists on the particular aircraft. Once the passenger self-identifies to you, you must ensure that the information is recorded and properly transmitted to personnel responsible for providing the accommodation.....
(a) omitted
(b) You must provide an adjoining seat for a person assisting a passenger with a disability in the following circumstances:
(1) When a passenger with a disability is traveling with a personal care attendant who will be performing a function for the individual during the flight that airline personnel are not required to perform (e.g., assistance with eating);
(2) -(4) omitted.
(c)- (d) omitted.
§382.83 Through what mechanisms do carriers make seating accommodations?
(a) If you are a carrier that provides advance seat assignments to passengers (i.e., offer seat assignments to passengers before the day of the flight), you must comply with the requirements of §382.81 of this Part by any of the following methods:
(1) You may "block" an adequate number of the seats used to provide the seating accommodations required by §382.81.
(i) You must not assign these seats to passengers who do not meet the criteria of §382.81 until 24 hours before the scheduled departure of the flight.
(ii) At any time up until 24 hours before the scheduled departure of the flight, you must assign a seat meeting the requirements of this section to a passenger with a disability meeting one or more of the requirements of §382.81 who requests it, at the time the passenger initially makes the request.
(iii) If a passenger with a disability specified in §382.81 does not make a request at least 24 hours before the scheduled departure of the flight, you must meet the passenger's request to the extent practicable, but you are not required to reassign a seat assigned to another passenger in order to do so.
(2) You may designate an adequate number of the seats used to provide seating accommodations required by §382.81 as "priority seats" for passengers with a disability.
(i) You must provide notice that all passengers assigned these seats (other than passengers with a disability listed in §382.81 of this Part) are subject to being reassigned to another seat if necessary to provide a seating accommodation required by this section.
(ii) You may provide this notice through your computer reservation system, verbal information provided by reservation personnel, ticket notices, gate announcements, counter signs, seat cards or notices, frequent-flier literature, or other appropriate means.
(iii) You must assign a seat meeting the requirements of this section to a passenger with a disability listed in §382.81 of this Part who requests the accommodation at the time the passenger makes the request. You may require such a passenger to check in and request the seating accommodation at least one hour before the standard check-in time for the flight. If all designated priority seats that would accommodate the passenger have been assigned to other passengers, you must reassign the seats of the other passengers as needed to provide the requested accommodation.
[text omitted]
(d) As a carrier, if you wish to use a different method of providing seating assignment accommodations to passengers with disabilities from those specified in this subpart, you must obtain the written concurrence of the Department of Transportation.
