Instead of "light", we would say "light-hearted." We could also describe him as not taking things seriously. However, in your examples his behavior can't really be described in one or two words if you want to convey the fact that he was laughing inappropriately in a serious situation.
He behaved inappropriately; he laughed and made jokes when everyone else was being very serious.1. He behaved (adv.)
He joked around at the meeting, which irritated everyone who was trying to have a serious discussion.2. He (v.) at the meeting.
He is very light-hearted, but in some situations that behavior can backfire on him.3. He is (adj.)