Business is business and friends are friends. Your boss is never your friend.

So, it's a question of ethics. You only listed one thing that might be seen as unethical: leaving him in a lurch with too much workload to handle himself without enough notice.

My advice is to take the new job, if it truly is a secured offer and the money is really there. If you get the deal signed, talk to your current boss asap and fill him in and offer to train a new hire for as long as you can.

It sounds like he's a decent boss, but to work as a DC in a multi-disciplinary setting is not an offer that comes around too often. If you explain your motivations perhaps you can leave on good business terms.