We took it to the vet at the RSPCA. She said that if I left it it would be put down because it has flu and is only 4 weeks old. So I bought him back home. He's on antibiotics and he has some high quality food. She wormed him and gave him a flea treatment and warned me he mightn't make it so we've called him Captain and he's quarantined from Rosie.
Oh, and he might have ringworm too so the boys can't touch him until we see if his bare patch grows or shrinks.

All that aside he's actually quite lively and playful and cute so if we get him through the next few weeks he'll be a great candidate for adoption.