I really was hoping to get some additional information from you before giving an answer but I do want to give you my thoughts.
Often this type of situation is the result of submissive or excitement urination. We usually ask customers to rule out medical causes for the urination such as a urinary tract infection or disc problem but you have already had him to the vet and do not indicate any medical issue was found.
You can try behavior modification to try and eliminate this problem. You can read of some techniques here.
There are medications which have been found to be effective for this problem and you should ask your Vet about Proin for dogs over 6 months and Imipramine for those younger. I've found that obedience training gives dogs self confidence and helps cure this problem as well. If you have not had him obedience trained, you will want to do so as once he is more confident, he will be less likely to urinate when greeting other people.
The following site is helpful in helping owners train their dog. Be sure and click on the link to the page on obedience at the bottom. and links on subsequent pages leading to detailed instructions.
Be sure and ask all visitors to try not to get him overly excited as well. Have them greet him in a normal tone of voice and not pet him until he is calm or sitting down. That will help if excitement is a trigger. Definitely talk to your vet about Proin to see if that will help the situation.
I hope this information is helpful to you. If you would like any additional information or have more questions please don’t hesitate to press the reply to expert or continue conversation button so I can address any issues you still have . If you do find this helpful, please take this opportunity to rate my answer positively so I am compensated for my time.