How do I teach my puppy I am the boss without being mean to him?

What a puppy understands is consistency and leadership. In a pack you will always find a leader and a pecking order from the leader on down. Dogs expect this and if you’re not the leader, they will try to be.
The perks of being a leader are that you get the best spot to sleep in. (Keep your puppy off beds and couches); you get the best food and eat first (never feed your dog off your plate or while you’re eating); the leader walks ahead of the pack through doorways, down halls, or anywhere that they cannot walk side by side. Once you show your dog in terms they can relate to that you are indeed the leader, they will happily follow your lead and instructions. If you slip up and let them think they need to be the leader because you aren’t acting like one, they will try to lead, and you will not like the results.
Puppies like children, need you to define their boundaries for them and to enforce the rules so that they feel safe and cared for. Please don’t confuse and frustrate them by failing to do your part.