I know it's hard (it was hard to convince myself I need one when it became time for me and my mom still claims she doesn't need help - but I've just gotten to the point where I request assistance for her without asking her) but may I suggest a frank talk where you address the fact that asking for assistance doesn't mean he is weak, not independent, etc. But rather it's help that is offered so that he can stay independent and to make his journey less tiring for him. Using assistance doesn't make him less than in any way. And I would say this whether he was transferring at FRA, LHR, CDG, AMS, or anywhere else. You just never know when there will be long distances, long lines, people rushing that knock you down etc. When you request assistance, there are different request levels. For example, someone recently was complaining because they normally are fine but their plane was at a remote stand and required stairs to board, so I let him know that there is an assistance level of "can walk, just need help with stairs." There's also can walk but can't walk long distances - as in I don't need a wheelchair at the gate, but assistance to get from gate to gate (which at many airports will be a cart, but not necessarily).

I hope you are able to find a solution your father will accept.