Shirley wrote: I feel really guilty leaving him behind when I take Bosco out.
Have you thought of getting a pet stroller? This means you can take Trapper out with you, he can ride along like the Queen, enjoying the change of scene, and if he's well enough get out in the park for a little potter around. I got one when Oliver had to have 4 weeks crate rest for his spondylosis, and then had to start slowly back into exercise, so that he could still come to obedience shows and visiting friends with Aled and me. Mine was about £40; it's quite sturdy (you can get more expensive off-road ones), folds up like a child's buggy, and even when Trapper no longer needs it, it's a very useful thing to have to lend out to others in similar situations (as Oliver's fine, mine's out on loan at the moment). Pity you're so far away, otherwise you could have borrowed mine!
Kate, Oliver and Aled
PS When Meg had CHF and was my only dog, I used to pop her into my bicycle basket and take her out for a country ride, with a few minutes sniffing round a field on the way!