He needs time, Going out is worrying for him and potentially embarrassing due to toilet and mobility/pain. It's a big effort, so if possible, take him out on a sunny day to somewhere he likes nearby so it's a short trip, (Father and Daughter time) and something he may enjoy and can look forward to doing again soon, once he knows its possible. Make it as easy as possible for him. Obviously you can't be your Mum but you do have that shared history and something in common to discuss.
Make it a special treat if you can. A trip down memory lane or a coffee and cake stop.
Regarding bereavement, our lost partners are still with us, have a chat with him and remind him that even now, if he asks his late wife a question, he will already know the answer as though she is still there. I do this, it really is comforting. Talk about the days when you were a child, and they took you out etc(roles reversed now). Kind wishes to you both.