I had the same thing with my son at the same age,I kept being reassured by men ,that when he goes to senior school and sees girls ,all will change, he was also not doing his teeth,he is such a lovely lady, people will not be able to get to know him due to the B.O..
I tried everything subtle, hints,buying him his choice of toiletries,and play acting that there is a smell and I hope it's not me,and his dad and pretending to check each other!
That didn't work,it got so bad ,I was in a shop with him and he reached up for a magazine,and it was strong and others noticed ,as his mum I was embarrassed for him and me.
I didn't want it to be from someone else and in a public place,as he would have been devastated!
So the only approach was direct as nothing else worked!
As his mum I thought it should be me who said it,he was very offended!! Very!!! But I explained that if I smelt ,I would want someone who loves me ,to think enough of me to tell me.
And I would tell his dad if he did and he would tell me,out of love and wanting to spare embarrassment.
It helped a lot ,I had to keep nudging him a bit,and reminding him that he wants to make a good impression,and he did get it, he around a deodarising body spray he liked,and the used it,so I found the best way is direct ,I have a good relationship with my granddaughter and I'm sure you have too,as a nan who loves her,you are looking out for her,explain this to her ,and take her shopping so she can pick up some nice smellies,make a day of it,look at the makeup,have a bit of lunch .
She will thank you for it later!