This.
It sucks at times. But it was part of DH's job. Its difficult to say to an important foreign client, ooh its getting late and my wife needs me to call. That says to them they aren't the centre of the known universe and your priority. Unfortunately whilst your wife should be the priority in practice, in trips like this you have to effectively suck it up. Thats why you are on the business trip, not someone else.
The issue of you not coping back at home, is a separate one. Its an important one, but this isn't about whether its normal for your husband to be out late whilst on a trip to Germany.
You don't have to like it. You do have to understand its effectively part of the job which pays your bills. If its an issue then your husband needs to look for an alternative job in the long term. But short term, it is what it is. You have to deal with it.
DS was a little older, but not much, when DH was doing this. I'm glad hes not still doing it. DH is glad hes not still doing it, though he does miss some of it. It used to annoy me if DH didn't ring/text when he said he would, and I have got annoyed in the past. It isn't easy. But the job was a really good one and opened the door to what came next and was worth it. And honestly, its better at that age rather than at an age when the kids start to miss Daddy. You just go into survive until the end of the week mode rather than thinking about more.
But the OP was about whether it was normal for this to happen when in Germany on a work trip, not whether the OP's husband should continue to do it for WHATEVER reason. It is normal.