I hope I can help with your question and do message me if you have any queries regarding my answer/advice.
I am sorry Charlie is lame.
It is possible that there are two problems going on one causing the persistent lameness
and one which has caused an acute swelling in the foot.
To examine long term lameness and altered limb position it is sensible to perform x rays and examine the limb under anaesthesia. This will allow a full orthopaedic examination of the limbs and joints. It is important to rule out cruciate disease which is where the ligaments supporting the knee joint weaken or ultimately snap. If this area is OK then x rays of the pelvis will help identify any abnormalities in hip joint development (dysplasia). The severity of the dysplasia will determine whether surgery is a viable option or more likely long term anti inflammatory, joint supplement and physiotherapy/weight management will be needed.
At the same time the now smaller foot swelling can be examined. Foreign bodies are common in between the toes and usually if still present resurface quickly especially if antibiotics have been used. The surgery to explore these lumps is very minor and leaves a small superficially wound. If there is no overt evidence of a foreign body and so exploration is not thought appropriate, the limp can be removed or sampled so that a pathologist can identify the type of reaction that is occurring (resolving inflammation or if there is a local mass/tumour) again this should be straightforward.
So I think the priority is to determine the lameness using x rays and palpation and at the same time the lump can be assessed.
It is indeed possible that there are/ have been two problems occurring at the same time.
Good luck and I hope he is OK
Dr Dan Makin