First, please do not put him in with the girls at all until he is six weeks post neuter. Right now he is still fertile so could still get them pregnant.
Secondly, bonding is a one time thing and seen through to conclusion on one day. You pick a day to do it (when he is post neuter safe), and then put them all into a neutral territory bonding pen with no hides and just a pile of hay and leave them to see if they will get on.
If sounds as if you have been putting them together and separating them repeatedly. Aside from the fact a non-neutered or not yet six weeks post op boar should never have access to sows, putting them together and then separating repeatedly causes a lot of stress to them. Piggies do not do these kinds of playdates. To them a meeting is a full on bonding session which gets repeatedly interrupted, they never form a relationship and then have to start all over again the next time you put them together. This is stressful and frustrating to them.
Bonding is a one time event and after initial introduction, piggies should not be separated again unless it clearly fails.
You must not put him directly into the girls cage - bonding must never be done in a cage of one the parties - they will see that as a territory invasion and could cause a fight and mean they refuse to accept him.
The bonding needs to be done in totally neural territory - somewhere none of them see as their normal space (bathroom or kitchen floor for example).
Put them in the neutral bonding pen and leave them for several hours. Once acceptance and rough hierarchy has been establish in neutral territory and you are satisfied they are getting on, you then clean out the cage they are all to live in so that it no longer carries the scent of any of the piggies who used to live in there. After that you move them all to the cage together and leave them together (do not separate them after this bonding day as it means that the process has been interrupted and they are not bonded). At that point you can add hides into the cage. It will then take two weeks for them to fully sort out their relationship
Thirdly, when bonding day comes, it is entirely normal he is going to rumble and mount them. It is a very normal behaviour. A boar going in with sows is going to produce a very strong reaction from him initially and once they are bonded, the sows first season with him permanently living with them is also going to produce a strong reaction from him. The girls may also have a stronger season than normal for the first one after they are bonded.
However, he is always going to mount them when the girls allow it, he is always going to rumble for the rest of their lives together. It is just what they do!
Bonding and Interaction: Illustrated social behaviours and bonding dynamics
