P.M.
Chemistry between child and parent varies so much, and children themselves are so individual in their development, capacities and understanding, that you could either give your son a real boost toward potty success, or confound him and create additional layers of negativity for him to overcome in the future.
Kids with communication delays are often much slower to potty train. But if his sister is showing the way, your son might actually enjoy following suit. I would say give it a try and see what happens. If it causes stress or frustration, back off and give him a chance to mature a bit more.
Here's a wonderful, extremely informative website you might find helpful. It gives a few variations on"readiness" checklists, plus tips on various training strategies, the best ages to start them, and the advantages and disadvantages of each approach: http://www.parentingscience.com/potty-training-tips.html