Advice needed on new aggression in dog

[Anonymous]

We have a male neutered lab, We adopted him when he was 10 mos and he has now just turned 2 ( so we think)

Up until about 2 mos ago he was the most goofiest, loveable dog around. Friends with everyone. He is still amazing with people but has started going for new dogs we meet.
We walk about 3-5x a week off leash in the woods with other friends and their dogs. He is fine with them, but anytime we meet a new dog or a dog he has met before but hasnt seen for a while he will be friendly at first and then out of nowhere start attacking the other dog.

Possible things that might be affecting him,
We do have a baby in the house now, however baby is 10 mos and this didnt start when the baby first came home.
One of the larger dogs in the group was very aggressive with him several times, no outright attack, and that dog doesnt walk with us anymore.
The woods are 5 mins from our house and he thinks its his territory
He is 2 and trying to be manly.

If anyone has any suggestions in how to get this behavior to stop I would really appreciate. I am consider a dog trainer but not sure how that would help, Ideally I want him to stay off the leash in the woods and burn some energy.

[Anonymous]

Bring a can of compressed air (like from office supply store) and squirt it whenever he starts the negative behavior. He will hopefully learn quickly that's not okay.

[Anonymous]

How is he in other situations? Remember that aggression is rewarding in and of itself, so don't let this happen. Look for signs of arousal: a wagging tail doesn't always mean friendly. Is his ruff up? Is he staring? You have to redirect before this point.

[Anonymous]

Op here, At first he totally looks friendly, his tail wags and his whole body wags, his head goes down, like he wants to play. Then he goes stiff and stares for about a second, then he attacks. I have managed to catch him a couple of times right when I see the stare because I know whats about to happen.

The compressed air is a good idea, I will try that.

[Anonymous]

Op again, he is not aggressive any other time, He is great with the baby and all people, I have never seen be in any way dominant aside from this.
We do have another dog, a shepherd mix, and they get on well together.