I know he's in solitude but I can't seem to find him. Anyone know where he is?

Check the Castle yard. When in the Market place, look west, go up the ramp, past the Blacksmith (who'll be on your right)...you'll pass through a large archway into a yard where men are practising combat. Look for the man with the beard (no helmet) instructing them.

Thats where I found him.

I am looking for him too, I am wondering if the fact I have completed the Stormcloak quest line has removed Aldis?

Anybody know?

I did the stormclaok questline as well before I ever went to solitude so my first time there was during the battle, but now that im doing quests here, I cant find captain aldis anywhere? Do you kill him during the battle, I would hate to have to chuck half my game progress because I killed a questgiver before getting the quest.

As far as I found in wiki the stormcloaks questline will make him disappear, since you are replacing the imperials with stormcloaks. the only solution I found of course is for console on pc player.placeatme 00041FB8 to spawn him, and I think he should have the quest options for you. If you arent on PC im sorry but I don't think theres any option for you.