Awhhh I wish my mom would give me a little brother or sister lol
Let him know he is still an important part of your life. My kids are 20,18,16, 5 and I am pregnant with my 5th child. I went through what you are going through when I had my youngest. I always made sure to do things with them before she was born. I am doing that again with this one. Just let him know he will never be replaced. Good luck!
Im only 23 myself so havent gone through it myself but it sounds like its a teenage phase, i know hes 19 but hes probably use to having your full attention and knows that he wont have that when the new baby arrives..I personally think that once the baby comes and he gets use to the idea and sees that he hasnt been pushed out and that you still love him just as much as you did before, he will be fine. He might be a little creeped out (i dont mean that in a mean way) that hes mum is having a child after such a age gap, i think i would have been shocked if my mum had another baby when i was 19 lol..Im sure he will come around :)