Thanks for all of your help everyone! The vet watched his behaviour and had a feel of the lump for a little while and doesn't think it's bothering him at all. She said it's very likely fluid filled and gave us the option of putting him under and taking a sample from it. He's older now and not as a strong as he once was so I decided it would be a better decision not to do that incase it stressed him out too much. She asked us to keep an eye on him to make sure that he's okay and doesn't seem to be in any pain or discomfort. So I think he's alright
An operation to remove it is the only other thing she suggested, although there's the worry that it could be connected to a blood vessel and cause more issues. As he seems okay it doesn't seem like it would be necessary anyways. Thanks for your advice everyone, Buzz had a vegetable toothbrush chew after his vet visit and a few extra sunflower seeds. So he's very content.