So maybe others had problems with this as well or maybe I'm just dumb but...

After you ascend as a witch and if you go Infernalist you can get the 2 points to get the hellhound but it was (maybe still is bugged) and it wouldn't summon him.

Simple fix: Go to your skill gems and hover over the hellhound spell summon thing and right click to activate him and boom you have him with you now.

Hope this helps someone out there!