I am an H1B Visa holder and I am trying to find out, besides my tax being deducted by my employer, does he pay any additional taxes with respect to my employment. He is telling me that he pays almost 10% of my hourly rate as employer tax? Is this true?

Yes, that IS true.

The employer MATCHES the FICA taxes you pay. You pay 7.65% and so does the employer.

Further, he pays either a state or a fedeal unemployment tax of about one-half of 1% (this is the federal rate; state rates vary).

I suppose it's possible that several of the high-tax states (California, Massachusetts, New York) may have unemployment tax rates in the 2-3% range.

Further, there are disability insurance charges that several state require the employer to pay on each employee.

BOTTOM LINE: Yes, it is VERY possible that your employer is paying 10% of your pay in taxes.