I agree there's absolutely no need to buy a suit that will never be worn again! I think he'd be fine in plain black or navy trousers (school uniform type, you can get them cheaply from ASDA or M & S if he doesn't already have them), with black school shoes and a shirt (long or short sleeved, white or coloured/patterned). He could borrow one of his dad's ties or bow ties if he wanted to jazz it up further, but in my opinion that is smart enough for a small boy. My own boys strongly resisted wearing anything more fancy (and wouldn't tuck in the shirt but at least it looked like we'd made an effort, which is the point!)

Can anyone advise me about what a 7 year old boy should wear to a wedding. 

My girls will wear smart dresses, that is easy, but I am not sure what the form is for boys of his age at a formal occasion. My in-laws are quite old school and it is their side of the family but I don't particularly like nor do I want to spend money on a suit or something that will never get a second airing. 

And that's before I have started on my own outfit hunt 

Thanks in advance for any suggestions.