Was there a clause in his contract to suggest he couldn't have a favorable opinion for Castro?
If said contract actually included a related clause, would it be legal?
As for if it is a 1st Amendment Thingy or not, there is also article 9 of the Bill of Rights that may come into play:
"
Rule of construction of Constitution
The enumeration in the Constitution, of certain rights, shall not be construed to deny or disparage others retained by the people. " 
.