Scooby Has A Lump

Scooby has a rather large lump (an inch across) under the skin on his left hip.
It's somewhat solid feeling, but not totally and I am able to shift it a little back and forth with my fingers.
I just discovered it a couple days ago.
He doesn't indicate any pain with it. Doesn't look like an abscess. He still jumps up & down from the bed and chairs although in the past several months he has been moving slower when walking or going down into his catloaf position. I was thinking he maybe has a touch of arthritis. But as you all know, lumps are worrysome.
We have an appointment with Dr. Rich tomorrow morning, so keep him in your thoughts and purrs.