Could you make new post with more photos i.e. the tree itself, closeup of trunk, leaves arrangement, & fruits position on the tree? For now I could only speculating maybe something in the Cassimiroa genus(?). If this is Cassimiroa, I can’t tell if this is a species or a cultivar hybrid the included link can help you with propagation method. Also if this a grafted tree, seedlings from its fruits will not be the same as its parent tree. Again with out more details to help with the ID, I’m only guessing. Best wishes! http://edis.ifas.ufl.edu/hs304