We wound up snuggling our boy to bed until he was 3 1/2... It was a very, very slow process for us--the whole transitions thing. Anyway. We started by snuggling him to bed and putting him in his bed. Then we moved bedtime preparations to his room (I know, this is so obvious...). Then we snuggled in his bed to get him to sleep. Then we snuggled for a while and sneaked out. Then we snuggled for a few minutes. Then we sat and read by his nightlight (3 feet from his bed). Then we read in the hallway. Finally, we started sneaking away from the hallway. Oh--and this took us almost a year. (Sorry.) Finally, at 3 1/2, we had a new baby, and I ASKED him to sleep through the night without crawling in with us. Using a rewards system (stickers, things from the One Spot at Target) we got him in his own bed, all night, and we mostly sleep through the night with him. We only did rewards for maybe a month, and after about a week of nightly rewards, we did "Now you have to go TWO nights...then THREE nights..." and so forth. Eventually he lost track of counting and we stopped the rewards. If only #2 could learn that... Good luck; I know how frustrating sleep issues can be!
Oh!!! I see someone has mentioned sensory stuff--my son sleeps better when I put him in lighter pj's and he can lay covered with a heavy quilt. His grandma made him a quilt from old jeans, and it's way too warm--so we turn the house way down, put him in light pj's, and bundle him up. He really does seem to sleep better that way.