"the good reason to have a late S election on 2553 is the client didn't know that until now"

What does that mean?  Did the client THINK it already was being taxed as a S-corporation?  I don't think just 'not knowing' what is required is a reasonable cause, and I suspect the IRS could deny it for 2018.


"I think S Corp fit b/c his income is triple than last year"

That doesn't necessarily mean a S-corporation is better (especially with the §199A deduction now).  However, more importantly, it goes back to what the client thought was happening.  If YOU just decided NOW that a S-corporation is a good idea, then the late election is NOT valid because the client was not treating it as a corporation.  Again, the late election generally only applies if the client has acting like a corporation.  Was a S-corporation extension filed before the March 15th due date?  If not, it seems even less likely that the client has been treating it as a corporation.