Ist it ok to take my cat for a walk?

My cat is a year old of as of last week. He has solely been an indoor cat. Recently my boyfriend and I have brought him outside (holding him the whole time) for like 5 mins a day to get some fresh air. He loves it and even whines at the door when i get home because he wants to go out. I don't want him outside alone, because there are other cats outside that I am sure are diseased and malnourished (plus I don't want to see him go missing). I am wondering if I bought one of those cat harnesses if it would be ok and even remotely possible to take my cat for a walk?