Ok, first things first, solicitor letters are just that... letters. He could choose to ignore them, reply to them or sending flying to the bin in the shape of a plane.
So... my advice, please ask a solicitor to include a date by which you want a reply, and saying what will happen if he doesn't reply by then (ie. We are expecting to hear which days you want to spend with the children during x holiday, if we have not heard from you by then, we will proceed with our holiday bookings in the understanding no contact is planned for those dates.) But keep in mind that he still can choose to ignore the letter, however, if he decides to take you to court for "blocking contact" on such dates, you have evidence you tried to arrange contact but he didn't reply. But keep in mind, that he can choose to ginore the letter anyway.
Don't be tempted to put more pressure on him sending him more solicitors letters as he may be trying to drain the financial resources you have allocated for solicitor fees to restrict the amount of legal advice you can get at a later date.
Another tactic you may try is to push for mediation, and if he doesn't reply in several weeks put an application forward to court asking for a residence/shared residence order AND a contact order in his favour. That should get him to sort something quickly.
Bear in mind however, that the fact that he is not getting involved in trying to establish a contact pattern is bad news. Some parents are not bottered about having regular contact with their children (and to involved in enjoying their new singledom) so keep in mind that if he doesn't want contact, no court order can force him to have it. 