What is the standard for defamation?/False accusations to avoid paying unemployment ``

I have been working for a small co. for the past 3 yrs. and I have always been an exemplary employee.
One day I made the terrible mistake of being honest with my employeer and I told him I was applying to be a police officer with a local police dept. and he fired me due to this reason.
After a couple of weeks, he rehired me ( I couldnt find another job, he couldnt find someone to fill my shoes). We had an oral agreement that I could stay with the company on a temporary basis till Oct. 31, and that I would take road trips to deliver equipment if I had proper notice. I had to be back in town by 5 pm because I was attending the police academy and attendance is mandatory.
However, after our agreement, it seemed that my boss did everything in his power to make me want to quit, like causing a hostile work enviroment by pitting other employees against me..etc
One day my supervisor came up to me in the middle of the day and demanded that I deliver some equipment to a customer or I would be fired. I told him I couldnt do it because I had school and I wouldnt be back in time. The supervisor then told me to go home for the rest of the day. I then called my boss and told him of the situation and he advised me to stay at work until he got there. My supervisor wasnt happy that I went above him and talked to the boss.
The next day I was given the option to either quit or be fired and that if I was fired, he would block my unemployment because I was insubordinate by not taking the road trip and he said we never had any agreement that I mentioned earlier.
Now I applied to another company and my potential employer called my old boss for a reference right in front of me. My boss then proceeded to tell him that I was insubordinate, which isnt true and that I was in the police academy looking for work as a police officer...well, I could kiss that job goodbye or any other job that I may apply for.
My question is: Is it legal for my old boss to say such things to potential employers? Does he have the right to give them personal information like the fact that I am in school? Also, do I have any defense against this alleged insubordination with unemployment?
Please help!