Additionally, although OP says that he is "connecting" at NBO, he is not. He has purchased two tickets for himself and thus has no protection should his BA flight into NBO arrive late, while his father will be taken care of by BA.

The only hope here is to price out changing the father's ticket to abandon the onward segment and then purchase a new ticket for him from NBO. That would leave a single LHR-NBO which ought to be upgradeable. But, it also creates the same problem for the father as separate tickets already creates for the son.