Aww. I wouldn't worry about him telling his friend - it's a hotly debated topic at that age so he won't be the first or last to drop the bombshell.
Our daughter started secondary school in September so we sat her down in August and told her The Truth. To say she was devastated was an understatement! It was the end of the world, adults were mass liars and conspirators and Christmas was ruined FOREVER.
I really hadn't expected her to take it so bad and ended up sugarcoating it....St. Nick was real, grown ups WANT to carry on the tradition for him so "The Christmas Spirit" was very much real and nothing would change on Christmas Day, etc, etc.
She was pretty unconvinced until I said, "Look at your Dad; he hates doing stuff and hates spending money even more so do you really think he'd voluntarily and happily take himself off to spend a small fortune to fill the sacks? No.
It's the Christmas Spirit, see?"
I told her she was grown up enough to know about the Christmas Spirit and she was now part of keeping it alive (and secret!) for the wee ones. Next day, all forgotten and calm restored.
I came away from the drama vowing to be honest with son (just turned 9) if he asked because it was the 'lying' that upset daughter most but figured he'd have another year or two believing at least.
Unfortunately he asked me outright a couple of months ago and I told him.

I started to go into that whole spiel ^^^^ Xmas spirit, St Nick, yadda, yadda but he was a different kettle of fish altogether. He cut me off, not a bit interested in that waffle, and laughed his head off. "I can't believe it was YOU. Wow! How'd ya keep so quiet, eh? Flip me. You're like ninjas!"
He was dead on with it and you could see the cogs whirring to see if he could somehow get more out of it. He actually said, "Heh, that's why I didn't get £10,000 when I put it on my list last year. Here, I know what I'm putting on my list this year: your debit card!" No flies on that boy.
He left the room happy but I admit I cried.

Too soon for me and there's no one in the house this year who believes. I even phoned my husband at work to tell him A Terrible Thing Had Happened.
But we're all happily in denial now. Still talking about Santa coming and we'll do the usual - leaving out a mince pie and a carrot and all that.
Every child is so different. All you can do is play it by ear and follow their lead. I wouldn't worry that he's telling his friends and not you. They like to sound wise to their friends but he might not be ready to admit to you - maybe, like myself as a kid, you kinda know but don't want to let on to your folks in case you don't get anything.
Personally I'm no good at waiting quietly and would have to bring up a conversation about Santa and take it from there. But I'm a pushy sort so maybe it is best to leave for now and go along with it all for now. Make the most of the pretence if you know what I mean.