I have been reading Healthy Sleep Habits, Happy Child by Marc Weissbluth and he suggests that sleeping through the night for a while and then beginning to wake up twice a night around four months is normal. He explains it scientifically in his book so that it helps to understand why. He also says that by 9 months he should no longer have to wake up during the night anymore (not to dishearten you, either, because it could happen before then!). So far, that has proved to be true for my son. He began sleeping through the night (8 p.m. until 7 a.m.) at 6 weeks, and when he turned 4 months exactly, began waking up twice a night. He's now 7 months and is getting up only once a night now (sleeping from 7 p.m. until 4 a.m., then gets up at 6:30 a.m.). My son was also having a terrible time getting to sleep at night, but after reading this book, he goes to bed great most of the time (he has a bad night here and there, but not many!). Generally, he says if a baby gets overtired by not having enough naps (getting enough sleep) during the day, your baby will become overtired and the body releases adrenalin making it very hard to fall asleep. My son is now on a rather strict nap and bedtime schedule and we also have a very regular bedtime routine, but it has really helped! It took a week or so, but he's doing so much better now.
I also agree with a previous commenter that said he may still be young for solids. I know more and more doctors are saying to wait until they are 6 months if you can to avoid stomach problems and allergies.
I recommend Healthy Sleep Habits, Happy Child. It's very informative and my son is sleeping so much better now during the day and night. The first part of the book is very dry and tough to get through, but after that you get into the "meat" of the book and has been very helpful for me (it's age specified, too, so you don't have to read more than what's applicable to your son's age now).
I hope this helps! Good luck!