My childhood dog has lymphoma and it's getting worse

I don't really have much else to say. I'm entering my 20s and he's had a very good life, however I've not experienced this big a loss before and I'm scared of how I'll feel. He's happy and still has energy but he's getting worse.

I have to return to a university outside of my hometown in a few days and there's no way to guarantee he'll be here when I'm back. I don't really know what to do.