It sounds like you have your heart set on him no matter what...please take note of what Dennis says, hopefully you'll be able to keep your emotions out of this decision and do what's right for both of you. I'd print Dennis' comments out and make sure the trainer is aware of this. To be honest, it appears that you are justifying a lot of his issues, which is another red flag that you may be making a decision with your heart rather than your head. On the other hand, the trainers should be well aware if his physical condition and know whether or not he's suitable. I'd still defer to Dennis though over the trainer.
I'm not sure how far he's walking but at 2 years old these walks should not make him walk like an old dog. We took Ben out yesterday (he's 10.5) for about 2.5 hours along with Brooke and he was fine, not dragging at all. I'm not sure how long the walks are, but it makes me wonder if it's pain, not his physical condition.
Good Luck with whatever you decide.