Hi there. There is a sleep regression at 18 months that causes EWs and NWs and nap resistance of just the kind you have described. Usually it pays to continue doing what you are and not making any significant changes to routine during the SR and things sort of get back on track by themselves.
Just in case it is something else, is he teething at all? Do you offer meds before naps/BT? If it makes a difference you could also try a dream-fed medicine at about 2am. It may help the wakings.
I wouldn't be keen to advise you to cut the nap even further so young. My DD is fairly LSN and she had a dip in sleep during the development and SR at 18 months, but went back to her longer nap and no NWs once it had passed.
What do you think?
There is a support thread here if it helps you to talk about it:
18 month sleep regression support thread - Part 2