seniorminnionn, I would say, as long as you are happy with the quality of workmanship, like you said pay him but don't use him again (always your choice).
From the sound of it he is a good engineer just not up to date with the current regulations (and lets face it , they do change daily).
He could have put this boiler in last week and , next week regulations change and its out of date!
As for the condensate, yes it will eat metal and concrete but it only has the same ph as tomato ketchup and I have seen what it can do to a concrete path after a couple of years.
Damaging but not catastrophic and certainly not enough to undermine a house.