Quite honestly, the sleep on the back thing is a strong suggestion. My son refused to sleep on his back from about 2 weeks on. He associated back with play, so I had to put him on his stomach. He is now 7 months and healthy as can be. Besides, when my husband was little, that is exactly what his mother was told to do. Just because you are told to put him down on his back, doesn't mean he is still safe, it only REDUCES the risk of SIDS.
Your health is just as important as his. If you aren't able to take care of yourself, you will dry out and be of no use to your little one, yourself, or your husband (not necessarily in that order).
Yes, you are right about the sleeping with you thing! By this time he should be sleeping through the night, but since he isn't, here is a suggestion. Have Dad get him during the night. You little guy is smelling your milk when you get him, so he thinks it is time to eat. To break him of this, you need to have Daddy get him instead of you. Dad needs to hold him, calm him and then explain that it is time to go to bed and then lay him back down. (I know he doesn't understand what you are saying yet, but it will help for when he becomes a toddler and dows understand.)
You also need to transition him to his own room. You and your hubby need your room and space back. Your relationship with him is more important than with your son. He will learn from you regardless and he needs Mommy and Daddy to stay close in all aspects. Don't worry about neglecting him, you won't!
It is okay to let him cry for up to 20 minutes. I usually check in before then if he sounds absolutely irate. Just note that you are getting close to a growth spurt time, too, so he may need a midnight top-off from time to time.
I hope you find this helpful!
May God bless you!