Hi,
so I just got a youth keeper. He looks like he’ll be alright, though is obviously Class E, so my question is:

How do I best develop him so that he reaches his potential as efficiently as possible?

Obviously there is no way I can play him in games in the Premier League.

Nor would anyone loan for him, so no point hoping someone would loan for him and play him in any games.

Is all I can do is put Coaching Investments to 10?

It is the best you can do to start with. Once he reaches a reasonable level I would try to loan him and see if he improve by playing matches (or you wait until the report tells you he needs to play matches). If you can’t loan him when he requires playing matches you can even play him in the field instead of as a GK, it still works (I know, unrealistic but can be pretty handy).