Yes it is okay to change a 7 year old boys diaper in public.
If your 7 year old boy needs to wear diapers then he will need his diaper changed.
So you can and should change the 7 year old boys diaper anywhere that it's needed including in public.
I've seen some older kids getting their diapers changed in public.
You can change your 7 year old boys diaper on a park bench, on the ground, in a public bathroom, on a picnic table etc.
I a few years ago I saw a boy of around 7 getting his diaper changed at a rest area.
His mom undressed him on a picnic table and changed his Pampers diapers right there.
Shen then left his pants off and he ran around playing for awhile in just his Pampers Cruisers diaper and shirt and shoes.
Then they had a picnic at the picnic table and he was wearing just his diaper, shirt and shoes.
After that they got into the van and he still has no pants on.
He seemed very happy running around in just his diaper and shirt and shoes.
I would also be happy if I could do the same.