Low immune system and Cold. Please help

A day ago my friend has cought cold and it happens quite often, it stays for almost 2to5 days and then it cures itself but after 2to2.5 weeks it happens again and the cycle continues. Actually he has a very very...low immunity power like if he spends a bit more time in water he'll catch cold that's why he doesn't take shower daily and he has dust allergies and these are also responsible for that cold and it also causes a minor chest pain. As he doesn't take bath regularly he gets dirty and catches cold but if he takes bath regularly then because of his low immunity power he'll become sick again. And during this time he sneezes a lot, he has mucus inside he coughs a little but it doesn't come out. Sometimes his nose gets stucked but most of the time it runs, his liking for food ends up totally. He is a very thin kind of a person, what ever food he eats that shows no effect on his body and if there is a little progress then this cold kills that and makes him thiner. Doctor Please Help🙏