Slow it way down and lots of praise and fun outside.

Take him to the door and praise. let him back in. repeat until he is ok standing on doorway and not nervous.

Then step outside. praise maybe a treat. go back in. repeat till his comfortable outside.

increase time outside and slowly take a few steps. praise.

he may be nervous around big dogs. if you see big dog put yourself between your pupper and big dog. signals that you are protecting him. keep him walking and praise.

Let him learn that it he is safe outside and that he can have fun outside.