Not possible. Everyone in a single PNR must have the exact same itinerary. Without pulling up the exact fare filings, IAH-YYZ is probably constructing based on two one ways anyways, so from a price stand point, you're probably coming out the same if you book a OW with you + dad and OW return with just you on a separate PNR.

Edit: just tried doing a dummy booking since I'm too lazy to pull up fare fillings right now. Got to love the weird interactions continuous pricing can produce, as two separate one ways are pricing lower than a RT on the exact same set of flights.

(these prices mean nothing for your specific itinerary as they were random dates 7 months from now)
Outbound priced as 425.55 alone
Inbound priced as 307.18 alone
Round trip priced as 805.65