If it's his personality, he may always be a bit fearful of strange situations. But you can help him to gain some confidence by carefully watching his stress levels and be sure to move on/away when you see him starting to get anxious. Talk calmly to him, don't cuddle or try to comfort him when he's upset, this will only solidify his 'idea' that there is something to be afraid of.
When I rescued Annie B she had been in a cage for 5yrs as a breeder at a puppymill. EVERYTHING was new to her from grass to traveling in the car to being out in the public. I started slowing taking her out, talking calmly and continually to her. If I saw her starting to react badly at a situation I'd just say in a calm happy voice, "Ok Annie, enough of this fun. Let's go get a biscuit!" She'd focus on the word biscuit...which she learned in like an hour
We'd turn and go away from whatever was starting to stress her and as soon as I saw her stress level coming down I'd stop and give her a biscuit. Let her catch her breath and then turn her back towards where we'd just come from. If she continued to stay calm, I'd offer her another treat and then usually go in another direction. After a base of trust was built I'd take her back towards the stress and usually she'd do much better.
It's a long process, but with patience and love you can help him to gain confidence.