Dogs who are missing a leg require little special care other than a healthy dose of common sense. Protect the remaining three limbs. Avoid games and activities that have him screeching to a halt or spinning on slippery surfaces. Leg stumps can be sensitive, so place him gently on to a soft surface when you work with him. Watch for signs of tiredness- bigger head bobs, sitting more often, slowing down, head lowered, working harder to catch his balance.
Dogs don't live with regret so they tend to compensate for this kind of loss amazingly well - just observe him to see what he can and cannot do. If he can chase a squirrel, he can keep up with you. If he can sit on a dime for a treat, he can do so with equal speed without one. Watch him as he moves freely about the house, then judge his abilities against that. Keep up the praise - you set the tone. Show him you have confidence in him and he'll have confidence in himself.