4 1/2 months is still too soon for your sex life to be back on track, in my opinion, after having a baby. There is so much to contend wth, so many new things to adjust to when you have a baby - your b/f is probably finding he is exhausted both physically and mentally. Take it from personal experience, I don't think mine came back until my son was a year old. But it WILL come back in time, and to something like it was before. He will eventually be more adapted to the routine with the baby, and so long as you're both interested in your sex life, there are ways to keep it alive after you have a child - you can still have a good sex life. After all children are not up til we are, you can still have romantic evenings when they're in the land of nod !! It will just take time and patience. It's unlikely he's gone off you, he's just got a lot on by the sounds of it.
Welcome to the word of parenthood, school, work and being tired! I personally think you also need to give your body time to heal. I wouldn't pressure him, everything will fall into place....heck, once a week is pretty good for people with children (and people without children now a days).