Is it permissible for a son to reclaim his father's share that was sold to his brother (the son's uncle) for a small price, by right of preemption (shufa'a)?

As long as the father sold his share to his brother willingly, the sale is valid. Pre-emption (Shufa'a) is the right of a partner to acquire the share of his co-owner from the person to whom it was transferred for a financial consideration, at the price agreed upon in the contract. It is a condition for the one claiming pre-emption to be a partner, and pre-emption is not established merely by sonship or brotherhood.