can someone please help

So from freshman year of college until last summer my bf gained about 20-30 pounds. It's never a year and 9 months later. I know people say you gain weight freshman year and stress but Im starting to wonder if it's maybe something else. Stress wise he is usually very carefree. My bf was never huge at working out and he always ate a lot more than me and never super healthy (but never complete junk food). When he's with me I eat very healthy. I know he sometimes eats late and I'm trying to stop that habit but he has eaten much healthier than he was. For the past few months he has eaten more vegetables, more water and made some changes ect.  I didn't expect him to change immediately but fought maybe he would lose weight to at least look it (we are long distance and I'm with him now) And I've noticed no difference. I should add that he wants to lose weight since his self esteem is horrible I'm not making him I'm trying to encourage him😊. I know he will think I'm crazy for suggesting a blood test or something but can this be something else? Like maybe his weight gain was from a deficiency or thyroid issue or something! Any advice?! I love him and want him to be more fit first and healthy I could care less about looks I don't want him to have any future health issues. Anyone relate?