Welcome on board
if you'll excuse the pun
I had to re-read your message at first - I couldn't believe a cocker could get to 5 stone. It's amazing he lived to get there. That's about 2 and a half times what a cocker should weigh. So glad you mamaged to resuce him and that he's now on the right road.
Now he's a more slimline model he'll be bursting with new found energy although he may need reminding that he's got it;D Cockers love to be outside sniffing around and running about. Inside, they love mental games and then lots of cuddles. Try giving him a Kong stuffed with suitable food that won't break his diet. You'll find that will also help with maintaining his weight as he'll have to work for the food and get it slowly.
A Buster Cube or treat ball is also worthwhile looking at. They taked the dry kibble and by rolling it around the dog releases bits of kibble. Can keep a dog amused for ages.
Good luck with Benji and it would be great to see some pics - a before and after shot would be brill.