I've started the Purity Quest where I have to lead Valkis to the tomb and cure him of being a werewolf. I accept the quest start leading him but as soon as were out the door he just turns around and walks back inside. I go in after him. I ask him to follow me and he says I've already got a follower. I got him to fast travel with me to the tomb once and on arrival outside the tomb- he just turned around and started walking back to Whiterun and refuses to follow me. Please can someone help me. I couldn't find anything online about it and nothing on YouTube

You could try going to one of the Inns and hiring a mercenary, then part ways with them to see if that frees up your follower "slot."

I've never run into this problem. Hope the above or something else clears it up for you.