A person can control their religion, and that's illegal to discriminate against. A person can also control whether or not they're in a union, for example, and retaliation from an employer based on that is illegal. "Is it something that can't be controlled" isn't the test for whether or not something is considered illegal discrimination in employment.

Which, also, none of us know why he's overweight and it's frankly irrelevant. There could be any number of reasons for it that aren't controllable or aren't currently under control, including mental health conditions and hormonal issues. He shouldn't need to give employers his medical records before they decide whether or not to comply with a request to exclude him and only him based on his appearance.