I do agree with Seren about the "speak truth without malice"....he (and every child) is entitled to love their parent....no matter what.
My children's father was/is an unsavory personality, to say the least. Now, that my children are grown...they have all told me, at different times, that they recognize and appreciate that I never spoke badly about their father...even though they can, now, see that I had plenty of room to do so....
Lol...there were plenty of times that I had to stuff a dish rag in my mouth to keep my bad thoughts to myself, in front of the children....