No-one should interfere with another person's equipment. Blocking numbers is easy enough on an Android phone and not telling him you've done is fairly simple.
The downside of doing what you want to do is that he will find out because a call from that number will still show on his incoming calls list and shown as a blocked number. He'll soon figure out who blocked it and that's probably going to lead to the end of the relationship.
Still want to do it?