Try to locate the book The Family Bed, it helps you to understand why or our primal instincts, this helps alot. We always at about eighteen months moved our kids to a trundle bed right next to the bed and during the day stored it under the bed, and you could just use the crib mattress or any mattress,or a thick piece of foam rubber, always use a mattress pad of some sort, this really helps the transistion and EVERYONE sleeps. also maybe making it soft and cuddly where your son feels not so open, the same idea of swadling a newborn, you could also have him wear one of your t-shirts or cuddle it for your smell, put it on his favorite bear.
I am the mother of six, ages ranging from 29 to 10 and have raised my two grandchildren as well, ages 7 and 10. With such a large family no one has ever had to be completely alone at night, usually about age three the shift would take place to sharing a room with the one of the older kids.
I always remind myself that even though my our children are younger than we are, they are still human beings just like us and we tend to like to have some one in close proximity when we sleep, it is a very primal instinct tied to our survival through the centuries.
If we nuture and keep our young close when they are young, they grow up to be some pretty stable, caring, wonderful people. I read this along time ago, I wish I knew who to credit for it, "We are not raising future children, we are raising future adults." That saying has always kept me pointed in the right direction while raising my 'future adults'.
ALWAYS follow the golden rule, treat others how you would like to be treated, and Lead by example, always respect your children in your dealings with them and they will learn to respect.
Great blessings to you, you are obviously very strong, and your job is very important because you are raising a future MAN - I was without a husband and 'fathers' three times during the raising of this family, and many times I was very lonely, but thankfully I was never alone - you are a very smart cookie, because you reached out. We have been given these little ones to guide, teach and protect. It is a huge responsibility - but I can tell all ready that you are going to be successful.