It is similar with Molly who is now 11. In an open litter tray she hangs her bum over the edge of it and poos on the floor. Every time. Peeing, she's fine. So we bought a covered tray. Liam will go in it (as well as the bath) but Molly won't, most likely because Liam's smell is in there. So she has to go outside. It's the only solution. Luckily we have sand in the backyard.
Maybe you can catch him peeing and quickly drop him into the tray?