Dog illness questions

Hi all

I've not been in this forum for ages but I have a question about senior dogs.

Bo the botany dog turned 12 this June, he's a mongrel (mainly alsation and whippet, yes he does look a little odd), he's also extremely active and lives with my mother. He is normally exercised twice a day for at least 30-60 minutes at a time, until a couple of months ago he and I would go on 2-6 mile hikes a few times a week (time constraints stopped that for a time). He started to get a lump under his foreleg about 3 weeks ago and then developed a limp. The vet advised complete rest for 2 weeks as he felt the fatty lump was from Bo being overweight (by a few lbs) and possibly arthritis.

As Bo thinks he's 5 and jumps around like an idiot, it's been a very tough 2 weeks for my Mom, poor Bo has been climbing the walls being unable to understand why he isn't allowed on his walks. He will not use the garden for toilet activities (his own rules) unless in complete desperation and then suffers terrible guilt after, so Mom has had to take him a few yards out of the front door to allow him to relieve himself. We're now into the 3rd week and he's had short 10 minute walks so as not to tax him too much but he gets very excited to be going out again and hard to keep calm. The fatty lump has gone

He is staying with me for a few days and I plan to take him 3 or 4 10 minute walks to get him out and about, he comes with me in the car on cooler days so he's not stuck in my house all the time. He's limping and stiff after he's been asleep, yet when he's out, he doesn't limp and appears to be in no pain at all.

My questions are - Could he be stiff through the lack of exercise this past two weeks? Am I doing right by taking him for little walks or should I be doing something else? Is arthritis in dogs something that suddenly appears, he's not had this problem before? Could a slight change in diet (he's not a greedy dog and has lost a lot of his appetite) or any suppliments like cod liver oil/vit and mins or similar help ease the stiffness?

Any suggestions as to how to help Bo recover as much as possible will be gratefully received. He's a very intelligent dog and when he gets bored he becomes dull and 'old', it's hard to keep his mind occupied which is why he's always been taken out a lot.