you can be certain of three things in life; death, taxes and the fact that babies sleep habits will not ever remain the same throughout their baby-hood... or childhood for that matter.
hes growing. his tiny tummy (about the size of his fist) fills and empties quickly. deny him food (breastmilk or bottle) and you deny him growth! also, any time a baby comes upon a developmental growth (rolling over, standing, walking) it will interrupt their sleep. we dont exactly know why this happens, but its so normal and so common.
blink, and he will have different sleep habits again, dont worry. it wont be like this forever, and you actually miss them when they dont wake up at night.
he can have solids, but not right before sleeping, this will only keep his system awake more because a digesting body isnt able to rest as well as a non-digesting body.
sounds like he might have allergy problems? constipation and crying to have bowel movements isnt normal. the most common allergy is milk but talk to your doctor. you can reduce your milk if hes breastfeeding or change his formula, but if you have to change his formula definately talk to your doctor first.
also give him more water. fiber is important to having soft stool, but it wont work without enough water. so keep that in mind.