Is this a very small business? I cannot imagine any boss who was not talking about his own, very small sort of private business who would be profoundly stupid enough to suggest to the person that they be about to cut their pay because they were now eligible for and drawing Social Security.
I'd be much more likely to suggest to you that another co-worker informed him you were drawing your social security, rather than that the SS administration contacted the employer. Or that he just happened to think of it and ask.
In actuality, as someone has pointed out, in regard to the pay cut, it hasn't happened yet. Your employer may be thinking that your amount of wages you can earn in a year will be restricted by social security, that he's doing you some kind of favor. Or he may be trying to flat out get rid of you, I don't know. If it's NOT age discrimination, yes, an employer can legally cut your pay, in the absence of a contract or union situation, as long as he pays you at least minimum wage for all the hours you actually work. And age discrimination would be interesting to call, is a bit tricky to show even in the most obvious of blatant cases which we don't really know enough about your situation to make any kind of real decision on it.