no, you don’t have anything to ‘worry’ about. At all. This is a tourist pattern, and Thailand really likes tourism. Like most countries. There is no risk whatsoever in denial of entry based on 4 months spent earlier in the year. He also Spent 5 months in his home country this year. You are just giving naive, alarmist responses. His past history will not cause him to be denied entry. IF there is a conversation, he may be ‘warned’ to get a visa before next entry