Kneel down. So your face is at his level, make eye contact, in a non distracting environment, and end with a hug.
Keep it Simple. Aspie's have difficulty sussing out what you mean sometimes. So don't be sarcastic, ambiguous or full of puns. Be direct. Mean what you say and say what you mean. Don't use extensive metaphors or analogies. Direct eye contact and calm pragmatics sure useful. Don't rely on body language. Do rely on spatial/visual don't use age determined expectations of comprehension or reciprocation in speech.