M.F.
Two french toast sticks and half a cookie sound about right for a kid his age. Also, if you'll remember with your first one, this is when toddler willfulness can be at its best. He'll refuse to eat just because he can, and because it gets his parents in a tizzy. Forcing food is not the answer. Simply offer appropriate, healthy food at appropriate times during the day (breakfast, morning snack, lunch, afternoon snack, dinner, and possibly a small bedtime snack), and don't say another word about it. Toddlers are pretty economical eaters -- they'll consume what they need to make it for about 2 hours, then look for more when their body needs it. And yes, they can go a day or two on not much more than crumbs.
Be mindful of the calories in beverages. Some kiddos will drink their calories, so they'll eat less. A glass of milk (especially chocolate milk, if you've gone that route) has a lot of calories, and so does juice. Limit the offering of milk to about 8 oz a day, and juice to 4 oz, or nix it altogether.
The toddler years is when bad food habits start because 1) kids need less at one time than we provide, so we think they're "not eating" or "wasting" food, and 2) they have quite a will, and use it.
As far as quantity, think of the size of his little fist for the main dish, and about 2 tablespoons of 1 or 2 other things. For example:
Breakfast: half a container of yogurt and two slices of a peach, a handful of dry cereal, with a sip or two of milk or juice.
AM snack: 2 or 3 crackers and a slice of cheese, or maybe a couple of mini-blueberry muffins.
lunch: one quarter of a turkey sandwich, 2 apple slices, and some water.
Afternoon snack: a couple of oreos and 4 oz of milk (which he might not finish).
dinner: 2 tablespoons of green beans, a couple of tater tots, and a chicken nugget
As far as his will, you just have to win the battle -- but do it without a fight. First tactic is to stay calm. Your job is to offer the food, but it's his job to eat it. Refusing to eat? No big deal, sweetie, "you must not be hungry, why don't you play with legos while the rest of us eat." If you'd rather him stay with the rest of the family at meal time, let him play with a toy or color while you eat. Also, ask him lots of questions (what animal do you like? can you count to 10? let's all say the alphabet...) or play games like I spy. This will help keep him occupied at the table. Or, if he's squirming and being a distraction at the table, offer these options: "You may eat nicely/join us at the table, or you may play in your room/the playpen, etc."
As far as ADHD -- you're right, a lot of kids with ADHD/ASD have food issues, usually related to texture. I've seen ADHD/ASD moms overcome those issues though with persistence about what is offered at mealtimes. If there's a particular food that you'd like to serve, say broccoli, you plan to offer it every day for about 2 weeks (literally, I've heard 14 days) and eventually they try it. The key is to offer it without saying anything about it. If they turn their nose up or fuss about it, just say "that's ok, you can push it to the edge of your plate/put it on your napkin."
Medically --
I'd focus on fluids if you're worried about dehydration right now. Maybe a fun cup with a straw. If he really is TOO thin, and losing weight, definitely make an appointment ASAP with your doc.
Sorry, this is super long.
Hope some of it is helpful.