This is the most stupid plan I've ever heard of.

Stupid, because it will not work out how you are hoping.

If you tell his mum I think you can almost guarantee that he'll find out that you told her.
Secondly, no mater your intentions I doubt that she will be pleased you gave him condoms.
Thirdly, if his mum gets on his case because of you, it could destroy your relationship with him. He'll never trust you again.

Dude, women don't think rationally. They think emotionally.
If you talk to his mum you will learn this the hard way I'm afraid.

This guy is your friend.
Communicate with HIM.
If you want to get his parents involved, then encourage HIM to talk to them about it.
At his age if he's not interested in talking to his 'rents then you telling them on his behalf will only damage your relationship with him and potentially get him into BIG TROUBLE.
Once adults get involved in kids having sex it can quickly escalate into a legal matter and once it goes there there's no turning back.

The truth has a way of coming out even if you don't mean or want it to. All the lies that you are building into your plan make it virtually impossible to succeed I think.

My advice to you is to talk to your yf. Continue to encourage him on a healthy path, but you aren't his parents... you can't make him do things the way you believe they should be done. Sometimes you will have to step back and let go of things that are NOT YOUR RESPONSIBILITY.

In my humble opinion, if you follow this plan your life is going to get really bad really fast.

Blessings bro
Cat.