M.C.
Diarrhea is a known side effect of amoxicilin. As long as you can keep him taking fluids.
At four months did he just start table foods? Its possible that the crabbiness is due to teething, and/or adjusting to the new food. My kids always had poop issues when they were teething. They also had them when they started new foods that didn't agree with them. Diarrhea can be a sign of a virus, it can also be a sign that your baby isn't digesting a new food well.