My pregnancy bible (your pregnancy week by week) strongly advises not to do any exercises involving laying on your back after 16 weeks because your muscles are relaxing so it is soo much easier to pull and strain them. But I wouldn't worry about baby just yet, its probably going through a rest phase. Have something cold or sugary and lay down and relax, if you don't feel anything at all for an hour call your doctor, but even if you just feel a kick or two it means babys alright.
If u gonna do exercise make sure its not related to ur abs or strainuous exercise it can hurt ur baby....count how many kicks in 10 mins after u drink a cold glass of water nd lay on ur side with ur hand on ur belly...
Any exercises to do with ur abs i would stop doing. Its probably not comfy for ur baby either. Try prenatal yoga or something lighter. For ur son, drink a large glass of cold water, lay on ir left side in a quiet dark room. If you feel less then 10movements in 2hours call ur dr.