Hm. Well I am married to a 3rd year surgery resident. We are waiting to have kids until he is in his last year of residency. That was just personal preference though. None of the other residents on his team have kids yet. In fact, we all got married while the medical person was in residency, not in med school. Residency offers more financial stability than med school and we were all just older and mature enough by then.
Anyway I honestly found med school a lot more demanding time-wise in some ways. It was very unpredictable bc he would have to study for exams ALL.THE.TIME. He stills studies now but not to the same extent that he did while in med school. BC of the constant exams (or impending exams) he was always mentally weighed down by work. Now, on the other hand, he can pretty much forget about work once he leaves the hospital (unless he is on call of course).
It will really depend on what your BF wants to specialise in. All specialties are different in terms of how demanding they are. For instance if he chooses something like anesthesiology, he might be able to do more shift work and not have to be called in for emergencies (like surgery requires).