I'm so glad you both have some ideas for me - I am happy to answer your questions

We are feeding him 3 meals a day, and 3 snacks a day. Usually we have the snack shortly after the meal so that there is a good gap before the next meal. Typically we have breakfast at 8, snack at 9:30, lunch at 12, snack at 1:00, dinner at 5:30 and snack at 7:00. He seems to have a lot of difficulty chewing up his food, but I think it's mainly disinterest. If we can get him to keep chewing he can do it but he often just get distracted and will let the food sit in his mouth. Today I gave him a snack right after lunch and he ate quite a bit - probably 15 cashews, a handful of raisins and some Cheerio-like cereal, along with an extra cup of milk. So it seems that he doesn't get full at meals, but doesn't have the focus to eat quickly enough to get enough food. We try to keep mealtime to about 30 minutes. The snack he had standing up at his toddler table. Part of the reason he ate so much was that his little brother was crawling around and interested in the food too.
I think I have a healthy relationship with food. I eat meals with the kids 95% of the time. It's a little tricky to get both of my children fed AND eat with them so I don't know if he sees me "enjoy" my food much, rather I am shoveling in a bite here and there when I can. I do talk about how delicious the food is though...and use a lot of positive reinforcement when he does take a bite/chew etc.. My younger son is even helping by clapping for the older one when he's doing well

We don't have a TV and he can find anything to distract himself - a car driving by, a scratch on the table...He doesn't seem angry when we tell him to take bites but he is usually very reluctant. Sometimes it takes a mini-bribe - we will withhold a drink of milk until he takes a bite, or withhold his favorite part of the meal until he eats some of the other food, or offer a bite of dessert (or show it to him) if we really need to get him going.
I was surprised when we pointed out to his doctor that he dropped from the 50th to 15th percentile that he wasn't concerned. He just said that toddlers are picky eaters and that it is normal for them to drop down at this age and that he would likely continue to follow the 15th. He looked at my son and said he didn't appear "wasted" and generally appeared healthy so wasn't concerned. He is also not concerned about his language development. We attended a language screening clinic put on by the local hospital and they have referred us for a full evaluation. It takes about 8 months to get in though. Basically he is saying single sounds and still doesn't complete even single syllable words. "Up" is still "uh" and "ball" is still "ba". He doesn't make any "o" sounds. "Uh oh" is "uh uh". He is missing some other vowel sounds as well. He's not putting any words together into sentences. He has quite a few consonant sounds. He's not very good at blowing - he doesn't make raspberry noises or blow bubbles. We got him a recorder and he can blow into that but it has taken a fair bit of practice. He hasn't gotten the hang of the harmonica yet. I think the doctor is a bit too laid back and not taking our concerns seriously.
I've been trying to find a good balance between foods that are difficult to chew (steak) and foods that are easy (scrambled eggs) since he needs the chewing experience as well as the calories. I've started using cream and butter when cooking for him, especially in the eggs as well as pancakes. He seems to like those foods reasonably well. We used to make him smoothies consisting of milk, banana, avocado and peanut butter and he liked them quite a bit. Like anything else though he gets turned off from them if he gets them too often, so I can definitely use some other ideas too. I am sure that his language development and eating difficulties are related. It may be a result of lack of muscle development or something more serious but he is making improvements so hopefully it is just muscle development! He definitely has a negative association with food. I'm not sure how long it will take for him to build a more positive association but can imagine it could take a while.
Thanks ladies, I'm looking forward to what you have to say!