We are arriving into terminal 3 Heathrow 20 minutes after our son and his fiancé arrive at terminal 5.
You mean you are SCHEDULED to do this. Flight times vary day by day and you could actually arrive before him.
And then there's the amount of time it takes to get through immigration and customs. Any bags to check?
Make sure both of you have flight information for the other. As well as a way to text each other. (Don't call as you are not supposed to use your phones at immigration.)
Whoever arrives first and finishes all the administrative stuff texts the other and heads off to meet them. When you get there, you text again to tell them where you are.
There is a big Costa Coffee outside of Terminal 5 customs. (I think it's Costa but you can't miss it.) You could meet there.
There are many shops outside of T3 customs so I can't suggest any one.
But honestly, with all the variables, I'd just meet at your hotel.