Hi
First, a growth the size of your fist is big! Is sounds like its ulcerating and getting infected. That can't be comfortable and may actually be quite sore. There's also the risk of the tumour spreading from his leg to elsewhere in his body. Sorry but although you love him you can't really do much for him. Its quite possible that a vet could operate and remove the growth. That's the best course of action, take him for assessment by a vet if you can.
Cathy Wickenden BVet Med MRCVS
Barton Lodge Veterinary Centre
1 Midland Road, Hemel Hempstead, Hertfordshire, HP2 5BH
Tel: 01442 216048 24 hours
[
www.bartonlodgevets.co.uk]