Hi T.,
My second daughter also had a very large head (at almost 3, her head is still really big for her size! Her sister calls her "my little blockhead," to which my little one responds "Pinhead!" ah, sisters...). For the first several months it was really hard for her to hold her head up straight. Our doctor said to give it time, since little babies have such tiny necks, and especially when they have a big, heavy head, that is a lot of weight for those little neck muscles to hold up! By the time she was around 6 months old and crawling, she could hold her head up fine and she has developed completely normally since then. If your pediatrician advised you to keep an eye on things, then there is probably no cause for worry at this point. All kids develop a little differently.
I don't know if you have one of those ExerSaucer things, but if he is able to hold himself upright a little, you might try sitting him in that for short periods. (My older daughter HATED tummy time with a vengeance, so that's what we did instead!)