I have four kiddies- two sets of kids that are both 15 months apart.
Tips? Although your older one will be young, try to involve him as much as possible with the baby... Have him bring you diapers, have him kiss the baby before naps, you can read with the two of them, have him "hold" the baby (you are actually holding the baby on his lap), but don't force anything. My oldest was a fantastic sister to her when the second was born (15 months difference). She was unusually gentle and so very loving with her, so we tried to get her involved with everything we could with the baby.
Now, The third child of mine was very loving to her younger sister(15 month difference as well), but had a lot less patience, so we limited the time they did things together.
Today my children are 6, 4.5, 2.5 and 1. They generally get along fabulously. Hardly any jealousy with any of them. They also tend to like most of the same stuff cause they are all close in age. The only thing is since they are so close, they get very deeply insulted when things don't go well- lots of drama:) They also team-up on their poor parents. They are PERFECTLY well behaved when alone (which is rare), but together they can get into some trouble while feeding off each other... They are VERY close- like twins (finishing other's sentences and what-not)!
I love having them close together, but I'll have to tell you making the transition from one kids to two kids was a really hard step for us at first... the newborn, then another baby that is also very needy, nap times, diapers galore, trying to breast feed while the other one needs you! It is mainly fun once you get the hang of two, and I wouldn't have my kids' age differences changed if I had a choice- It is a challenge, but I love it. And what situation ISN'T challenging when it comes to raising kids properly?
Well, congratulations and good luck and have fun- if you have any questions for me being quite experienced with the whole 15 months apart thing, right me a message!