For all the reasons already mentioned - I wouldn't bother with the RSPCA at this point.
Even if they would come out, you'll just get his back up, make him defensive and thats NEVER the way to change peoples behaviour.
The way to change peoples behaviour is to work out WHY, and see if you can amend or adapt it.
WHY does he behave this way toward the dog? What need or desire is he fulfilling for himself, by over feeding the dog?
Lets say he feels good, he feels kind and generous by giving the dog titbits and treats.
So there are healthy ways to give dogs lots of treats, my dogs get THOUSANDS of treats a week, seriously, it practically fucking rains dog treats in my house, I have dog treats in my pockets, in the car, in drawers, in cupboards.. all over the place.
However - those dog treats are suitable, liver cake, tiny bits of cheese, tiny bits of sausage, tiny bits of carrot, apple, and liver and garlic 'pills'..
Incredibly high value on the whole, but very low calorie wise, no sugar either.
They also don't get fed a huge amount of food from their bowls and what they do get is raw meat/meaty bones, so theres no sugar and colours and crap there either.
As a result, all my dogs are lean and fit (wish I was!), and they do mini training sessions all through the day so they are happy and motivated and generally obedient.
Is there any way you could encourage them to pamper their pet by feeding a better food, feeding the dogs food as rewards (just use the kibbles as treats), feeding from puzzle toys or chew toys that you fill up with food?
If he absolutely must feed human left overs, ask him to chop them into tiny pieces and scatter across a wide area of grass - then let the dog out to find them with his nose.
This activity is INCREDIBLY mentally satisfying for the dog, calming, relaxing, and they get a huge amount of pleasure out of it (its one of my most used 'tools' in changing dog behaviour where dogs need to calm down and get some 'job satisfaction' without being razzed up).
All these suggestions would meet his need to be generous to the dog, without the dog being harmed, AND if you can suggest and demonstrate these things diplomatically, without getting his back up and pissing him off!