On a package reservation everyone on the package has to have the same dining plan and tickets so if your package is for 6 nights and your son is on the reservation, his package will have to be the same as everyone else's.
If you keep your current 6 night package, the dining credits are pooled to the room so anyone in the room can use them after your son leaves.
There are some ways to consider if you don't want to pay for the dining plan for your son after he leaves.
Disney doesn't recognize split stays and considers them 2 separate reservations so if you do a split stay and want a dining plan for both stays, you will have to purchase tickets for both reservations unless you have an AP, qualify for military rates or renting DVC points in which case you wouldn't have to buy tickets and it would make it much simpler. For AP holders and military rate qualifiers you can book a ticketless package with dining. DVC point rentals do not require tickets to add the dining plan. If you have to purchase tickets for both reservations, buy tickets on the first stay for the number of days your son will need, but you will only be able to book FP+ for the number of days on this first set of tickets. Everyone else in the room can add more days to their tickets once you've arrived at DW. For the second reservation buy the minimum number of days required for the dining plan and save those tickets for a future trip. If you book the same resort and room category for both reservations, when you check in let the CM at the desk know you have a continuing reservation. While it won't guarantee that you will get to stay in the same room it might help. I don't know about DVC stays, but based on the PP's statement it sounds like you have to book 2 different room types.
You can do one reservation, but not include your son on the reservation. Add him after you arrive. All his meals and park ticket will be OOP and because he is not on your resort reservation he won't be able to take DME if you're flying into MCO. Also, you will not be able to make FP+ reservations for him because he won't be listed on your res.
If you want him to take DME with you, there is a narrow window when you can add him to your reservation, but without the dining plan. The dining plan has to be added no later than 3 days before arrival (or 2 days for DVC points stay). If you add him within 3 (or 2) days before arrival, call and add his info to DME, he'll be able to take the bus from MCO, but his food and park ticket will be OOP. Again, however, you won't be able to book FP+ for him until he is on your reservation.
Finally, currently there is a ticketless package called Better Together that is being offered, but you have to call and ask for it. It cannot be booked online. Very little has been posted about it and Disney hasn't publicized it so I don't know how long it will be available to book or whether there is a time frame for stay dates. Like the AP and military rate ticketless packages it follows all the rules for packages related to deposit, final payment due date, fees and penalties for changes and cancellation.
