I'm sure you'll receive many book recommendations so I'll skip to a few pieces of advice. Talk to your son as you light a candle telling him the flame is kind of like how much you love him. Linger on that for a while and call attention to its size before lighting another candle off the first. Now you have two flames, but show him that the first one didn't get smaller for the presence of the second. That's how love is -- it gets bigger.
Another thing that worked well for me is telling the older child that this new baby is his, that he (the first) was your baby, but this one is his. I found the older child took on a greater interest in the care and well-being of the squalling baby when there was ownership involved. "It's time to change your baby's diaper; want to help?"
Forestall jealousy: "We need to feed your baby, now. I don't really want to -- I'd rather spend time playing with you, but I suppose we have to. Do you want to go get a book for us to read while your baby drinks?"
My oldest was only 19 months old when the second arrived and 3.5 at the arrival of the third. He seemed so big by comparison that I surely expected more of him than if it'd just been him. In the throes of the certain fatigue to come, try to do your best to remember that he's still just a baby himself.
When exhausted, chant to yourself, "It's only a stage. It WILL pass."
Good luck.
C.