Perhaps it’s time he found his space elsewhere. At 25 years old you shouldn’t really be worrying what he is doing … if he is gay and sleeping with all the boys in town, or straight and sleeping will all the girls, that is up to him.
You can’t make any judgements re the ex GF – may be lovely / may be a bunny boiler. But whatever, she did have enough to hold his attention.
And he certainly shouldn’t be sponging off you
Does he have any “World of WarCraft” stuff?
Or a full sized Star Wars outfit?
As for not telling you about the GF , why should he? I didn’t tell my parents about every girl who showed me a good time !