How can I prevent this? Morgan is a good, clean boy at home but in other people's homes there is a problem

For example, he has twice peed in his groomer's home and when we went to visit a dog boarder, he peed in the hall. Tactics so far are to make him pee before we go inside and/or to take him through the house to make him pee in the garden/yard so that he knows where it is acceptable to pee.
Next weekend we're going to see our son's new house that he and his fiancee have just moved into, it'll be a long day so Morgan is going with us and I obviously don't want him marking the day in more ways than one

To add to the mix the future in-laws will also be there with their castrated labrador who Morgan has never met, although he loves labs whenever we meet them on our walks.