Hello and welcome!!
There are lots of people on here who have gone through the "Kevin the Teenager" months - if you do a search for teenager or kevin you will probably find lots and lots of advice..
The general advice is just to go back to basics - don't try and learn new things but constantly reinforce the things he knows but has forgotten... if recall is an issue, a long-line may help bring him back on track!!!
We are owned by Honey, who is 14 months old - her teenage trials have generally been more around behaviour than deaf-ears, but we are getting there - again with the help of a lot of advice from here!!!
Oh, and piccies are a must!!!
You wouldn't be a Dr Who fan, by any chance???