Probably 1-5% if he's the first employee. Less than a percent if he's not unless he got awarded more later or they have an unusual equity situation. 30% is founder territory.
I'm not sure of the exact number but it's somewhere between five and ten percent of the company, in incentive options (I'm not the first employee but a large stake was a condition of my agreeing to join the company in the first place, and they've added to that in two later rounds for various reasons.)
Are you sure they added to it? Even if you got more shares they might be a diluted percentage. "I'm not sure" often means "I've already been screwed and I'm not yet aware of it"... I hope not in your case, but if you don't know the numbers, don't take for granted that they are in your favor (this is the sort of thing you learn from painful experience).
Also be aware that (assuming you have investors) the deal might be structured so that the investors get their money back and your stock options are worthless. Getting a high salary in the acquihire job offer might be your best option.
What is your definition of "significant equity stake"? Are you talking 1%? 10%? 30%?