Hi!
I believe it is totally normal for a baby's poops to slow down a bit as solids are introduced. As for foods to 'make him go', there's a huge thread at the top of the Feeding Solid Food forum called "Laxative Foods" - my DS is severely constipated (screaming, rock hard poos, bleeding bum, etc) and I've had great success with prunes the last few days. But I wouldn't worry too much if your baby's not straining, uncomfortable, etc.
HTH
Sarah