"Can he stop me or worse, sue me?"
*** Of course, he could file a lawsuit against you since anyone can sue almost anyone else over almost anything.
However, contrary to the advice given above, I doubt that any lawsuit would be successful.
First, the Ohio "Uniform Trade Secrets Act" would not apply in your case, since NOTHING in your post meets the definitions in the Act.
"§ 1333.61 Definitions.
As used in sections 1333.61 to 1333.69 of the Revised Code, unless the context requires otherwise:
(A) "Improper means" includes theft, bribery, misrepresentation, breach or inducement of a breach of a duty to maintain secrecy, or espionage through electronic or other means.
(B) "Misappropriation" means any of the following:
(1) Acquisition of a trade secret of another by a person who knows or has reason to know that the trade secret was acquired by improper means;
(2) Disclosure or use of a trade secret of another without the express or implied consent of the other person by a person who did any of the following:
(a) Used improper means to acquire knowledge of the trade secret;
(b) At the time of disclosure or use, knew or had reason to know that the knowledge of the trade secret that the person acquired was derived from or through a person who had utilized improper means to acquire it, was acquired under circumstances giving rise to a duty to maintain its secrecy or limit its use, or was derived from or through a person who owed a duty to the person seeking relief to maintain its secrecy or limit its use;
(c) Before a material change of their position, knew or had reason to know that it was a trade secret and that knowledge of it had been acquired by accident or mistake.
(C) "Person" has the same meaning as in division (C) of section 1.59 of the Revised Code and includes governmental entities.
(D) "Trade secret" means information, including the whole or any portion or phase of any scientific or technical information, design, process, procedure, formula, pattern, compilation, program, device, method, technique, or improvement, or any business information or plans, financial information, or listing of names, addresses, or telephone numbers, that satisfies both of the following:
(1) It derives independent economic value, actual or potential, from not being generally known to, and not being readily ascertainable by proper means by, other persons who can obtain economic value from its disclosure or use.
(2) It is the subject of efforts that are reasonable under the circumstances to maintain its secrecy."
Source:
http://onlinedocs.andersonpublishing.com/oh/lpExt.dll/PORC/8e03/a71e/a803?fn=document-frame.htm&f=templates&2.0
Obviously, the sections D1 and D2 would be critical in any potential litigation on this subject. Simply, if you do NOT take any lists of any kind, and simply gain customers as a result of your own efforts, even using names and contacts that you may have developed in your business dealings, they would likely NOT meet those conditions.
Simply, if you leave his employ and do NOT take any copies of ANYTHING that could meet the above definitions, then you should be able to defend against any claims that he might make (if he makes any). Most courts are very lenient on the issue of your being able to make a living without 'constraint of trade'.