I'm not trying to be judgemental, but why is he in diapers at night? Is there a medical reason? Have you talked to his Pediatrician about ways to help him past this stage?
I'm a single Mom too, and I know it's hard to get through some of the major stages when you know you could really use the help. However, from reading your 'a little about me', it sounds like you've got a full house. I'm wondering how long you've had this giant brood? I commend you for taking on helping kids who need love and guidance, but is this maybe affecting your own little one? Again, just questions...
Is his father involved in his life at all? I know that can be more of a double-edged sword than helpful, but a male influence is SUPER necessary when it comes to boys and potty learning.
Does he have nightmare or any other issues?
I am in the process of potty teaching my son, since he asked the other day to use the 'potty chair' and it's a tough process. If your son doesn't have the ability to hold his urine at night by this age, I'm wondering if there is a muscle control issue. I don't know for a fact, but I read somewhere that most 'boys' develop this by around age 3 and should be able to 'control' their stream.
My last question, have you talked to him about it? Sat down and spent some time with him one-on-one to find out how he feels about it? I used to work as a teacher full-time, and this can be such a tough time for socialization and issues like this if not dealt with can cause huge self-esteem issues.
I hope you and your family can find a way to make nights easier for everyone.
Good Luck.
http://www.emedicinehealth.com/bedwetting/article_em.htm
http://www.nobedwetting.com/parents.htm
http://www.goodnites.com/na/expertadvice/Default.aspx