There were many things that Michael knew but couldn't tell Jace. Otherwise, it would be akin to treason!
With Lucas's identity, if he really wanted to harm the Huttons, no one in the family would be able to escape!
Like Jace said, it was because of his mother's help that Michael could secure the position of the Huttons' helmsman.
It was also the very reason that Michael had to obey his wife's request to drive Lucas and his mother out of DC and tell them never to come back.
Michael would never want to see his sons reaching the point of killing each other, so if he could appease Lucas by agreeing to some of his requests, Michael was willing to do so!
Even if Lucas wanted his life, he was willing!
But he couldn't tell Jace about these matters.