Willow bark contains the active ingredient of asprin. In fact, asprin was first developed as an extract of Willow bark.
Be careful with DMSO. It penetrates the skin very easily, but also makes the skin permiable. There are cases where people used it on their hands and then had contact with a normally non-toxic substance (because the skin blocked it) that then got inside and made them very sick. Safe as long as he's not going to contact any other liquids for a day or so.
When Rhubarb is having joint pain, we give her tart cherry juice with tumeric and milk thistle powder. 1/4 cup unsweetened, not concentrate cherry juice and one capsule mixed milk thistle and turmeric, pulled open and shaken into the juice. She didn't love it at first but it must be an acquired taste because within a couple weeks she'd do her excited dance when I asked if she wanted her juicy juice. She is syringe trained so giving her 0.5 ml is no problem, but sometimes I get lazy and mix it with her chop in the morning. I came up with that mix because:
Cherry juice - supports kidney function, helps with eliminating uric acid
Tumeric - Anti-inflammatory and pain relief
Milk thistle - Protects liver function and detoxifies