If you are 100 percent sure that he’s not sneaking food somewhere he might have some sort of hormonal issue or have thyroid issues. Now this is not an accusation on your boyfriend, but are you sure he’s not sneaking in anything? There’s a lot of people who do not know how many calories they are taking in.
And 50 kgs (100 pounds) that is an awful lot to gain without there being an underlying issue.
Keep us updated with the results!