My english mastiff with temper issues?

I have an English mastiff who is a little over 2 yrs old, a big boy, about 210 pounds, and when we got him (he was 2 months) my parents were very good with taking him to the dog park every weekend (where there are tons of dogs always there to socialize with) and taking him on walks, in the car, and to the dog store so he could be well socialize and a pleasant dog as he got older. But the weird thing is he never became a pleasant dog. For over the last year of having him whenever he goes out, even in the car, he will bark viciously at anything suspicious to him (such as people in certain colors, or anyone I guess that looks at him funny or something) and we can't take him to the park anymore because he will be vicious to dogs who do not provoke him at all, he will just randomly antagonize a dog to the point where we HAVE to leave because he is so big people get freaked out he acts that way. At home he is a gentle giant, he doesn't even bark that crazily when new visitors come over