Health insurance is compulsory here and it is private and nothing to do with your employer.
Perhaps he will assist you in finding an insurance company but that's about it, the rest is up to you.

In my opinion, what I would do, when you get an insurance company just insure everyone from the date you arrive, you are medically covered abroad here,with the insurance companies, however as your wife and son are not officially here could create a slight problem if they were really sick in the time they were not here, but if they were covered with an insurance company, it's always one risk less.

And if you insure them while they are not here, don't tell the insurance company that they are not here, let them assume that they are here.