To change a diaper on your 7 year old son in public you should take him to a public restroom or private restroom if possible.
Then have your 7 year old stand up or lay down and then remove the old diaper and wipe him up with some baby wipes.
Then simply place the new diaper back on your 7 year old and put his pants or shorts back on.
Out of respect for people and his privacy don't change him out in the open in public like a toddler or baby as he would likely be very embarrassed by that.
Take him to your vehicle or into a restroom and give him some privacy when changing his diaper.
You could change your 7 year old sons diaper on a public changing table in a public restroom.
A public changing room diaper change table can hold a 7 year old boy just fine.
Although if the changing table is out in the open then it would be best to take the 7 year old into a toilet stall to change his diaper.
At 7 years old he needs some privacy when changing his diaper as he would likely be embarrassed to have his diaper changed out in the open.
You could change your 7 year olds diaper in a toilet stall by having him stand up or lay him on the floor and change his diaper that way.
Or if he's capable then teach your 7 year old son to change his own diaper.
Also look for a disabled toilet restroom or a restroom that you can lock the door on if possible and then change his diaper in the single person restroom.
If that is not available then change your 7 year old sons diaper as discretely as possible.
You could also take your 7 year old son out to your car or vehicle and change his diaper in private that way.
Changing a 7 year olds diaper is just like changing a babies diaper or a toddlers diaper but you just need to give the 7 year old or older kid in diapers more privacy than you would a baby or toddler.