1. He does NOT need TD to file I-485, he simply needs to be in legal status in US (B2 is legal) and wait the appropriate time to file I-485. So, you will not be "more comfortable" when he is on TD, because it will be expiring shortly.
If he applies for TD now, since your TN is soon to expire, the will only grant him TD for that short time, and he will be out of status by the time he needs to file I-485. The letter is meaningless. if you do not have the TN approved yet, they will not grant him a TD longer than your current one.
He should enter on B2 (as he has always done) which will be granted for 6 months, then you should file I-129 and I-539 for TN and TD, and when these are approved, file for I-485.
You didn't say: Is he Cdn citizen?