If he had followed my advice, he wouldn't be in trouble now. This is correct. If you replaced "now" with "then", it wouldn't be correct.If he had followed my advice, he wouldn't have been in trouble now.
I think we can say then instead of now or say wouldn't be.
Thanks in advance.
. . If you replaced "now" with "", it wouldn't be correct.
If he had followed my advice, he wouldn't have been in trouble then. This is correct.