My son in law will be blowing the Shofar at the end of Yom Kippur. He practices about ten days before Yom Kippur. may he continue practicing on Yom Kippur between Musaf and Mincha?

No he may not. A shofar is considered Muktza, since a person might come to carry it four Amot in Reshut Harabim which is an Isur Deoaita. Rabbi Binyamin Bamberger