Deviating from your coaches plan?

I recently started working with a coach who moved me from a PPLxArnold to an A and B upper/lower/Rest split…I haven’t told him but I’m secretly adding an arm day on one of the rest days simply because I love training and I want to prioritize arms a bit more than what the split he gave me has me doing. Should I tell him I’m doing this? Keep doing this and watch my arms blow up? or simply just stick to his routine and stop the arm day?