I think it depends on how he plans to propose.It's just one of those things like other women shouldn't wear white, only the bride should else it will detract attention away from her on her special big day etc..
It can/does occasionally happen AT the wedding, some guy thinks weddings are so romantic, sees his girlfriend so happy at her friend's marriage and decided... he's going to get down on one knee and cause a scene just before the first dance or speeches or something.
Or you have things like the newly engaged woman who hasn't announced it to any of her & the bride's mutual friends yet turning up at the wedding not so subtly wearing her new engagement ring... someone at her table in say the reception clocks it and there is the same sort of drama; "OMG did you hear, Shazzer got engaged, yeah she's wearing the ring" "OMG I need to see it, let's go and congratulate her". 'Shazer' becomes the centre of attention for a bit instead.
I've been engaged before and it was just a private thing.
I wouldn't suggest doing it at the wedding or the reception. I agree that would be bad form.
If OP wants to make it a big celebration then he should probably do it on another day after the wedding if only because his own celebration wouldn't get the full attention either.