What a great success story with your grandad's rhubarb. I'd be worried about it in a 10in pot as it is a seriously thirsty plant and keeping it moist in so small a pot, especially once it has full sized foliage, will be nightmarous! I'd be inclined to transplant it, but do so asap, and keep it really, really well watered....and keep on giving it lots of TLC throughout the rest of spring and summer