How are you feeding him? Nursing or bottle? If he's nursing, you'll need to lower him a little so that he is level with your breast (although, if he's high enough to kiss I have a hard time seeing how you would nurse him at that level - if you are bottle feeding in the mei tai you would probably need to keep the bottle more horizontal so that his sucking out of the bottle is very intentional, and he's not getting milk dripping out of the bottle nipple while taking a breath - did that make sense?). Also, I loved my mei tai, but I could not figure out how to nurse successfully in it, so someone else may be able to give you better instructions.
Hmm..is the mei tai tied too tight? Honestly, I would find someone in your area who is very experienced with baby wearing and get a tutorial, or find a babywearing store and ask to show you how to tie it, and tell them what has been happening.
It's supposed to be snug, but not so tight that the baby can't turn their head, or look around a little. Instead of tying the straps behind baby's back, you might try tying under baby's bottom instead.
Is the body too long or too wide do you think? If too long, fold the bottom up once or twice (to desired height) then tie around waist. See if that maybe helps. I have to do that for one of my mei tais that is too long. I would try this first.
FWIW I always tie the mei tai so that it ties under the baby's bottom. Way more support that way...also less sagging.
Also, with your description of the head rest part... it sounds like the shoulder straps may be a little off... I don't know how you would fix that unless you do some messing around on a sewing machine.
Sewing, gardening, home birthing, co-sleeping, extended nursing, cloth diapering, baby-wearing, home schooling, attachment parenting busy mommy to dd1 (7), dd2 (4), ds (1) and two in heaven.