I know this is a very tough stage for you. But I've always found that if you just "indulge" this phase it passes quickly. Instead of constantly pushing him away...take the time to be with him constantly. We've called it "smother mother" and eventually the toddler does feel smothered and will separate on their own.
see when you constantly push them away they feel rejected and become all the more clingy. But if instead you allow this they feel accepted and soon get over their anxiety.
Hope it all works out for the best.