I'm assuming your son wakes at 10:30pm, 2:30am, 5am & 7am for the day. Naps at 9am for about an hour and again around 12:30pm for about 1 1/2 hours.
Well, it's a bit of both the "waiting it out" and trying some ideas. Here's my advice:
Try to keep him awake for his 9am nap and let him sleep the normal hour and a half around 12:30. If he falls asleep a little after 9am that's fine. The next day or two, try to keep him awake past the time he fell asleep before. Basically, it sounds like he still has his days & nights mixed up. Try to keep him awake as long as possible during the day. If your daughter does not nap the same time he does, simply let her play with him for awhile. It will keep him awake a bit longer during the day before he "passes out sleep". It worked for me! When you put him down at night, give him a cereal bottle that's heavy on the cereal. That should get him to sleep until he needs changing at 2:30am. Give him another cereal bottle if needed. If you are breast feeding, just add the cereal to the breast milk.
What does he eat for dinner before you lay him down? He should be into or just starting solid foods by now. So, if you haven't already give him something that will stick to his stomach like mashed potatoes, oatmeal, or rice along with a cereal bottle.
Also, he may just need to cry a bit. It's rough, but it works. He needs to learn how to get himself back to sleep without mom. Before you go and get him at night, wait a few minutes, then go in. Each day (or every couple of days) wait a little longer. Also, it may be time for a bigger diaper size.
Hope this helps. :)