If I were you, I would give them socialising time through the bars/mesh while he is waiting, so they can get used to each other. In my experience, the introduction is a bit less wild than if they met for the first time during intros.
Who you put first in a neutral introduction area doesn't matter, actually. I often start with a big flat plate of grass or hay in the middle, so nopig can get possessive. Ideally, there is nothing else.
With boars and girls, acceptance (or not) usually happens within the first 30 minutes. Ideally, the girls will let the boar sniff their bum and submit to the boar at least once, even though they are not in season or are not aggressive towards him. Telling him vocally that they are not in season and gettign their bum out of reach, but showing no other sign of aggression is also OK.
What is not good is completely ignoring the other party or any signs of aggression, which sometimes can build up very quickly if the wrong signals are exchanged. Have a towel at the ready! Please be aware that not all, especially older dominant, sows will necessarily accept a neutered boar.
If you can, stage the intros on the lawn; otherwise have your windows open - a neutered boar will spray the girls liberally with testosterone laden pee to mark them as "his", while the girls will spray back to tell him off. Please do not bathe - it is part of the bonding and you have to bear it until they have settled together. Boars can occasionally overexcited and lost the "on" switch for their brain - they will just hump anything from whatever direction. If necessary have a soft humping toy ready when things get a bit too wild and the girls too stressed out.
Immediately after acceptance has happened, piggies move into the dominance phase where the new hierarchy is being established. That can last several days. Move the piggies to their well cleaned and if possible rearranged cage once the initial excitement is over and there is a bit of a lull.
The first few seasons tend to be quite wild as the girls' bodies react to the presence of male pheromones. Do not interfere; it also a time for bonding intimately. Things will calm down a lot more over time!
Here is a list of signs of aggression in mounting order; Glynis has already given you the introduction link.
http://www.theguineapigforum.co.uk/showthread.php?t=28949
