brother what
closed ports are something that should be happening on your own network segment, it shouldn't be something your ISP has any control over at all.
They could block all inbound connections, but I think it's unlikely that's actually what's happening. And even then, I don't know if that would even affect a connectionless protocol like UDP (which is what most game servers would run on)
A lot of residential ISPs block inbound connections on extremely common ports like 80 and 443, but I've never heard of uncommon ports being blocked.
That being said, blocking inbound connections is a very different thing than blocking port forwarding. The latter shouldn't be possible; as you said, that's controlled entirely within your network.
It is possible that he just has an extremely basic router that simply doesn't offer it, in which case the solution would be to get a better router.
But I imagine VPN might work? I know people have used software like Hamachi to host without port forwarding
Better solution: if you have a VPS, why not just put the BL several on that?