Well, we didn't have other kids when we fostered, but now that our girls are adopted, I can only answer from what we would do if we fostered again, now.
I think part of being treated as one of the family means that things won't always be fair and equal. That's ok. Just focus on his needs and desires and try not to compare too much. There would still be unfair things even if he had always been with you because of age.
The therapeutic riding sounds great and can be "his" thing. I think as long as he has something to find identity within, it doesn't have to be exactly the same as your other kids. I also think 11 is old enough to talk with him about his feelings and help him understand and deal with it all. I would also mention it to his therapist (hopefully he has one) and he/she might ave some good ideas.