Boyfriend suck his thumb at 29 years old years old when sleep i no that's not normal at all is that a psychological problem? or mental ?

Harmless habits: Most human beings discover their own way of calming themselves and relaxing. As long as the habit is not harmful, even odd and unusual habits do not imply psychological or mental disorder. In this case, thumb sucking should not be harmful, unless you or other bed-mates are bothered by it. If you are going to abandon him because of this, then i suppose your boyfriend may want to try to stop.

Coat w lemon juice: That by it self is not indicative of a psychological or mental problem. It is a habit. If he wants to stop, he should try soaking his thumb in lemon juice before going to bed.