How Did You Train Your Rescue Dog Who Had No Training And Teach Him To Play ?

I have adopted a beautiful medium sized 2yr old rescue kelpie X (rotweiler I think). He is a very friendly, non agressive boy that is very jumpy and needs to lose a little weight- so he is heavy and strong. He loves other dogs and people but is too heavy,strong and jumpy to interact with them as he will knock them over. He doesn't know how to play which is a bit sad, am currently trying to teach him to play fetch and that tennis balls are fun.. He is responding well to turning my back and saying no when he jumps, in 5 days he's learnt to sit and learnt to sit while I put his lead on. Tell me your best tips and strategies. He's enrolled in doggy school that starts shortly. Thankyou in advance for your help and non judgement.