I have avoided posting here because I didn't want to say something that sounded rude. I will try my best to make this sound right and not rude.
I really do understand the frustration when a dog piddles all over when excited BUT I cannot ever imagine adopting a dog who has already been rehomed (especially at such a young age) and may have likely already had not such a great life. Taking on that responsibility of the lifetime commitment and being someone who that baby depends on and then even think of giving up only 5 days in for ANY reason. There are ways to deal with it. If you are his second home already at the age of 5 months and you then rehome him due to this problem to someone else who won't keep him for the same reason then eventually he will land in the hands of someone that will abuse him for it or send him to a shelter where he will likely be put to sleep.
Please please do all you can to either fix it or learn how to deal with it, be it diapers or whatever it needs to be. This baby doesn't do this on purpose or have any control over it and deserves to be secure in his home life.
My little girl Rosie used to do it quite a bit and now at almost 2 she is much much better with it. We have learned that if she is secure and happy in her surroundings she is less likely to do it now and if we think it is a time she may do it we learned not to reach down and try to pick her up or pet her until she is more settled and she now has it much better under control.
Hang in there and help him work through it.
