Mallenders is crusty thickened sore skin on the back of the knees on the front legs and Sallenders usually the front of the hock on the hinds. That really looks more like mud fever to me that has been festering away under long feathers for some time.
I agree with the others avoid steroid use if at all possible, the long term implications aren't good and can lead to weakened areas of skin that are even more susceptible to damage and sores.
It is also imperative if it is mud fever that the scabs have to be removed or you will never clear it up, it just gives a nice warm moist environment for it to multiply and get worse if scabs are left on.. Any cream that softens the scabs so you can gently remove them will do the trick. So long as the scabs aren't allowed to reform and you keep treating with any gentle skin cream they will eventually clear up altogether.
When you have eventually cleared all signs of the sores I would test a small area with pig oil and sulphur. If he doesn't react badly I would use it two weekly right through the year. It worked wonderfully on one of my old coloureds (some of you may remember Solly?) he always had very long and beautifully white feathers all year around. Even if you want to keep Sid feather free I would still have a regime with the pig oil and sulphur as it will protect him from further outbreaks of mud fever. If he does get Sallenders or mallenders you can also use the PO and S to soften the area and work away any rough scabby bits.
I think he is gorgeous by the way - you are very lucky to have him and I wish you many happy years together.