Quoting *KatteyBug*:" I think once they're weaned, it's almost impossible to re-introduce it. Especially with him being 2 already. :/ Maybe ask your doctor?"
Why do you think that? generally once toddlers see there siblings nursing they take great interest in it.
If he wants to nurse then feel free to try op, remember his teeth will feel very different then you newbs mouth, in the mean time the cup is a great idea. Im not all to sure what it will do for his delays but it will be a great immune boost for him. goodluck!