Working at the direction of your employer, you can assist him in collecting past due legal fees by any otherwise lawful means. You cannot, of course, represent yourself as a lawyer, but you can advice clients who have not paid that you are his legal assistant and attempting to collect on his behalf.
You can make phone calls, write letters, employ a collection agency and/or file lawsuits to recover the unpaid fees. All of this would have to be done in your employer's name, of course, but you can do a lot of it on his behalf. He would have to be the named plaintiff in any lawsuit and would have to sign the complaint, but you could prepare the paperwork and do the legwork to get it filed.
You should be aware of the federal Fair Debt Collection Practices Act, with which you would also have to comply. For the Fair Trade Commission's brochure on the act, see:http://www.ftc.gov/bcp/edu/pubs/consumer/credit/cre27.pdf
Your boss can give you authority to work out payment arrangements on his behalf, though any written agreement regarding payment of fees should be signed by him.
You cannot engage in any conduct which would be prohibited for him to do himself. You should be familiar with the Texas State Bar rules regarding attorney-client relationships, in particular Rule 1.04 regarding fees. You can see the rules at:
Good luck with this effort.