Unless there are unusual circumstances (like he is an accredited diplomat) then...
EHIC E111 is for holidays not for residents, he will be resident if he works here.
If he works here then he must be registered in the tax system here and liable to pay tax here, A company here is very unlikely to have anything to do with paying salary outside the tax system. He will need a "tax number" to do a lot of official things here like open a bank account.
As a tax paying resident he will have the same access to health care as all the other tax paying residents.
Personal insurance is not compulsory but a matter of choice and there are many different products and levels of cover.
