Its not going to be easy for him to be canned as long as:

1) He has completed probation

2)He didnt violate any laws with his posts.

3) He was not wearing his uniform or wearing any company ID or logos etc while making these statements\

4) He was not making any statements (verbal or written) while using the companys likeness

5) He was not using any company property to post, and was not posting on company time.

If any of these 5 took place, then he deserves to, and is likely to be canned, BUT as long as none of those 5 things took place, its going to be VERY easy for any attorney worth their salt to win this pilots case.

Remember, unlike the Air Canada pilot who willingly ID'd himself as such, this guy did NOT make any mention of where he worked, and it was only discovered after he (rightfully) caused controversery.