
We had a phone call from one family not long after their pup went home. The reality of having the pup had suddenly dawned on them and I think they panicked a bit because they weren't sure they could cope.
I made it very clear to them that for the pup's sake it would be better for them to give him back to me sooner rather than later, trying VERY hard not to pressurise them into a decision, and keeping the whole thing as calm as possible.
Well they sat down as a family and discussed it. They got a dog behaviourist involved who helped them raise the pup in the right way and they never looked back. The last communication from them was full of praise of their boy who they absolutley adore. He really has become part of their family and I'm sure they are so glad they decided to keep.
I did have one returned from the same litter at 8 months. I really didn't see it coming and was pre-empted with an email. Within a couple of days he was back and although was re-homed, 8 months old was plenty enough for lots of bad habits which needed ironing out. He went on to a great new home so the best was done for the dog in the end.
At 5 months your pup is still very much a baby so I would make them realise what is ahead re training/exercise/socialisation if they want a well behaved, happy adult dog. Make it clear you would be very glad to take him back, but as you have already done, offer your support either way.
Good luck