My son is a finishing contractor in Saskatchewan. He has,

My son is a finishing contractor in Saskatchewan. He has, since March 1, 2014 been working intermittently on a finishing contract for a 185 unit rental apartment complex.
His original quote for the project provided for work to begin March 1, 2014 and proceed at minimum pace of 8 units completed / week. This was the pace requested by the project manager and this, as well as other terms of his quote, were incorporated by the project manager into the contract he signed.
The project has been a gong show from the beginning, with poor site management and poor work from other subtrades requiring massive reworking so that the project is hopelessly behind schedule. My son has done everything he can to keep the project moving when he is on site, including moving materials and various other misc. responsibilities which were specifically excluded in his contract. When he is able to work his crew was routinely completing 16 – 20 units per week. He has repeatedly been told to report to the site because further units are ready for work, only to arrive and be able to only put in an hour or two of organizational work because necessary materials are necessary. He has other contracts around the province and on several occasions brought crews back to the city because he was told they would have another floor released to them on Monday morning. Finally, as a result of the lack of work and the problems he had taking on other contracts because he was trying to remain available to honour this one he had to lay off about half of his employees last fall.
He has completed about 75% of the project and has been paid for about 65% (less holdbacks). He and his remaining crew members are currently working on a project in northern Manitoba. He has just received a text from the site manager advising him that they now want him to complete the remaining work by the end of February (which would be a ludicrous impossibility even if he still had his whole crew). This is after he was, in late November, given an earlier deadline of December 31 for completion … since which he has been unable to work on the site more than 10 days or so due to units not being ready for finishing.
He has been advised that if he does not have his crew on site by Monday that they will get another contractor. He has also been advised that if he does not complete the contract on their new deadline he will not be paid for some his outstanding billings (about $30,000 outstanding invoices and another $10,000 ready to be billed) and, in addition, he will be charged a penalty for any “extra amount” they have to pay a replacement.
My son does excellent work and has gone out of his way to try to assist this project (to almost ridiculous levels, IMHO). To us it appears the site owners are in massive breach of the contract he signed and yet he has, until this week, come back to their site within a day or two whenever they have advised him work was available / required – even though several times they had flat out lied to him.
Can you give us any insight into his legal position / options?