Deca is exactly what he needs for his joint and back pain. 300mg will do for a start and may be all he ever needs. He will need test to add to that like you said, even just 100mg/wk will be enough. But 250mg is more fun.
Deca is the man right here, I have suffered a serious shoulder injury in my teens and altough its heeled I still feel pain when really training hard of lifting heavy, with the Deca its painless I can push beyond what I think my limits are with that little shoulder!!! Deca also helps if your wife / girl /parents / dog are anti juice, you just tell them you are purely using it for your joints...