I'd been holding out hope that Numelgon wasn't going to leave, and obviously nothing's confirmed yet, but after seeing it destroy Tierno's Kameil and Raichu yesterday I don't think there's any way the writers are going to let Satoshi keep it for long. At best, it'll just return for the League. I would sure as hell love to wrong, though.
As someone who was excited that for once Satoshi got to have a strong Pokémon before the post-League episodes (and since it's getting less and less likely he'll get a Mega Evolution...), the idea that he might release it is really disappointing. And I don't agree with Famon a lot, but this is the kind of thing I feel the writers would only do with Satoshi. Hikari got to keep Mammoo, Iris kept Kairyu, Citron will keep Rentorar, but Satoshi will likely lose Numelgon?
On top of that, Numelgon is a fun Pokémon with actual development so it'd be highly disappointing to see it go.