Weak with no sugar ...Camomile tea always helped my daughter and it's natural!
I gave my son Gripe Water, it works wonders! We also tried 2 oz of chamomile tea and that helped calm him down and helped him sleep.
How old is he? When my son was very little sometimes it really helped with constipation to lay him on his back, hold his ankles in the air with legs bent, and kind of rotate them in a "bicycle" kind of fashion to get some motion moving in his hips. Some light belly massage helps too to stimulate a bowel movement.
The other thing that always seemed to work (though you've gotta be using them already) was to put him in a brand new beautiful cloth diaper for the first time. Never failed - he always pooped in the new ones right away lol ;-)
My daughter is 3 weeks old tomorrow and she has gas issues. We bought gripe water and it helps a bit. Putting her on her tummy to lay down seems to help too.