Take the money off the table and take your fear of what the current boss may do and look again at which job is the most attractive.
No one knows where they will be in 5 months, 5 years or 5 decades, wondering if the current boss (if he is still there) will take you back may be unfairly shadowing the better job, it may not too but you'll never know and may well regret that if you don't consider all angles. You also don't know if you will want to or need to go back to the current job.
If you're going just tell the boss, be honest, tell him why, don't mention better pay and better conditions etc just tell him it's because it's what you really want to do but your time working there was good and may like to return again one day. If he's a decent boss he will have made up his mind about your future prospects before you've finished your spiel.