Think, for a moment, about YOUR rights. Not the convenience of your boss, or the disruption to the clients, but your rights.
Is your boss actually asking you to disguise your physical pregnant body? Is he asking you to lie about your pregnancy? Is he asking you to conceal the truth about your pregnancy?
All of the above are grossly unfair on you, overtly discriminatory and, I imagine, an illegal request. Unbelievable, in fact.
Don't do it. Get some decent advice from HR about your rights whilst pregnant. This must be dreadfully stressful, and your boss is so far out of line. What would happen if you didn't oblige him and just got on with your job, knowing that your pregnancy was obvious? Or if you actually informed your clients?
He can't take any action against you for being pregnant, nor for informing clients of that fact.
Get some advice, and please stop putting your own reputation and wellbeing at risk to appease your boss.