Please do post some photos of your baby
We'll do whatever we can to help.
It sounds like one thing your baby really needs is confidence. You can do a lot to help him build that through simple training. Start out doing things he already knows, praise and treat lavishly, then add more behaviors, exercises, tricks, etc., to the sessions -- slowly -- set him up to succeed and absolutely reward him, especially with praise.
Now that you've got him used to being praised and liking it a lot, use that reinforcement when he's out with you. When he sees a person (start with at distance) and doesn't react fearfully, reinforce that with the reward.
You get the picture
Confidence is the best way to help make your mastiff safe.
If he needs to add some weight, Satin Balls are great. There are several recipes on the web.
