Occasional unexplained laughing is common in autistic children — it can be self-stimulatory, a memory replay, or simple joy, and by itself is rarely a problem. Look for context: is he otherwise eating, sleeping, and engaging as usual? Note when it happens and what precedes it. Mention it at your next doctor visit; seek advice sooner if laughing episodes come with staring spells, loss of responsiveness, or regression of skills, so the doctor can rule out other causes.
Disclaimer: This is general information shared for parent awareness, not medical advice. Every child is different — please confirm decisions with your child's doctor or a qualified, RCI-registered professional.