This is just a matter of personal preference, but I have a hard time justifying a SAR on suspicion of tax evasion. This is because I don't know how the guy is going to file his taxes. (Now, if he's a borrower & I subsequently get his tax forms, that's a different story...)
Based on the situation you describe, if it's me, I'm building an evidence file. I don't think it's a SAR yet.
Opinions are Bartman's, not those of my employer. "A noble spirit embiggens the smallest man."