B.C.
Might be a very mild stomach bug (is there any fever?) or maybe if the weather has been very hot it might be affecting him a bit.
A sudden hot spell can make me feel a bit sick to my stomach even if I'm in air conditioning all day.
I'd try putting down a towel with a plastic bag or shower curtain under it next to his bed for a few days so clean up is easier.
Try making his last snack/meal before bed happen a bit earlier so it has more time to get past his stomach before laying down for bed.
Keep him well hydrated.