Your concerns are well justified - my concern is more that my son is a picky eater right now and it's hard to find something on the adult menu to share that he will like...you have a few options.
-Upgrade to Delxue plan. You will have to pay the difference between
DDP & DXDP on free dinning, but you do get so much more that you're sure to have plenty for your son. It inlcudes 3 meals CS or TS - your choice. The meals inlcude dessert & appetizer which is great because depending upon the restaurant the appetizer is like an extra meal and some appetizers are great for a kids meal.
Plus you get 2 snacks per adult a day - plenty for drinks, ice cream, yogurt, fruit, etc. for you and your son. (we're on DXDP)
-Choose mostly family style or buffet meals. I know it may be hard because you have another family to consider, but at buffets and family style restaurants your little one can share as much as they want off your plate. It's the most economical way to have a meal for your child w/o having to put them on the plan.
-Choose CS meals carefully. Some CS meals are basic burgers & fries, but others are unbelievable meals. Cosmic Rays in MK has an area with chicken and ribs - you can get BBQ Chicken, Ribs, potatoes a veggie, drink and dessert as a CS - plenty for you to share.
-Pay OP for your son. The kids meals are very reasonable and if you pay OP for your son when needed it will cost much less then having to buy him a ticket so he can get the free DDP.
-Bring food & snacks for your son. We ordered some stuff from Garden Grocer last year (DS was almost 2 when we went). We had indiividual apple sauces - great complimets to any meal. And the Gerber Graduate Raviolis are great. They come in easy to carry small tubs and are fine heated or not. Again a great supplement for DS meals. We also had some of his favorite snacks with us - goldfish, etc. Again, we also did this because DS can be picky and I wanted make sure he had food he would eat. Plus sometimes a child can't wait for their food in a restaurant and having somethinf for him to eat (applesauce) while waiting was helpful.
-Check allearsnet.com for menus of all the restaurants. It will help you choose places that have foods you can share with your son.
Depending upon your family and how you like your meals & such paying OP will probably be the most economical. I believe the difference between the DDP and DXDP is about $32 per person per day. If you think between extra food for you and DH (breakfast or whatever) and you son the cost will be over $60 per day then go with the DXDP. Remember, with DXDP every meal can be table service if you want. The value for us is we all like breakfast on vacation. Plus it's my son's favorite meal. With DXDP we can go to a character breakfast every morning - DS eats well, iit's a fun way to start the day and we're all fueld up for an active morning in the parks. Lunch is usually CS and then dinner TS. Also with DXDP you don't mind using 2 table credits for special meals - this year we're going to Mickey's Backyard BBQ whihc is 2 credits pre person. Plus you get refillable resort mugs which are good for mornig coffee or a cool drink at the end of the day. You do have way too many snack credits - but at the end of the tri[ we use left over snack credits to buy lillipops or certain individual Disney candies and such to take home. Some are good snacks for the plane - others are nice little gifts for kids and such.
We're going during
free dining as well, but we're staying at DVC so cannot get the free dining - so we're paying the full price for DXDP and it's well worth it. Last your our OP would have been like $400 more if we didn't have DXDP (we were 3 adults and 1 child under 3).
If you want a meal plan that you don't have to lay out 1 cent more ofr food while at Disney then go DXDP. If the DDP is more than enough for your and DH, the OP costs for your son will be very reasonable.