First of all, we never declare anything about pregnancy, unfaithfulness, sickness or other thing that can have a great impact on the questioners mind or health.
Let me try to give you some thoughts, seen from my life experience and not from any psychic stand.
You are very young, 20, and I hope I can congratulate you both with yours first child.
For most womans "parenthood" come as a natural thing in connection with birth.
For many males it comes like a shock, and they might need time to adjust,he (we) is nothing but "a overgrown boy", that has had a nice time with you alone.
Now a new citizen has moved into the home, who might cry a lot, who might have feeding times, at short intervals, even at night, who turn around all the routine you and your hubby had from before. This might give him some fear, or insecure on how to deal with the situation.
I am not saying all males act like this, but some might do.
I think you shall act as natural as possible, don't try to push anything on the sex front, don't try to start any quarrels.
Try to let him take his turn with the baby, it is now you must make the ground for how you want your hubby to take part in yours sons future.
If he takes no part now, it will be harder to make him understand about it later....be firm...try not to make arguments.
If you go around believing that your hubby has someone on the side, you will start to beam mistrust towards him.
You might not say anything,direct, but such thing will be picked up, because there will always be given comments one way or another.
If he feel your mistrust, this might lead to bad feelings, which also"lower the sexual activity" for the partnership.
I am not saying you shall go around wearing blinders...you must always evaluate your marriage/partnership, and it is always important to talk frankly about thing that bother you, because thoughts can burn one mind very hard, if allowed to grow.
So try to relax, enjoy the child together, and hopefully all three of you have a harmonic life again soon...I know I would
Good Luck to you all!