OP, this is his business. That, from the sounds of it, he’s worked hard to build possibly at the expense of his family life. He’s not just a regular boss/MD. He’s the owner and boss. You will rank lower than his company, even if you do great things for his company, he will still put the company before you.
Time and time again in workplaces women sleep with their bosses. It always reflects badly on the woman, rarely the man. She’s the one who gets a reputation that she carries with her to the next role (and the role after that). She’s the one accused of sleeping her way to the top. You’ve put in good work, it’s clear you’re proud of the rock you’ve done for the company - do you want that tarnished because you sleep with him? Even if a relationship built and you married the guy, the other people in the office would still link your performance to who you are sleeping with.
I don’t understand why you think I’d be treated so badly by him just by sleeping with him, surely some perfectly good relationships start like this.
You’re in deep OP. You don’t just want a fumble. You’re convincing yourself that good relationships can start this way. So sure, some good relationships start this way. However, he devotes himself to his company, he’s a workaholic with a divorced family. He’s probably not going to want children with you, if that’s something you want. He’s not going to give you share in his business if you were to get married. He’s not going to work less and spend more time with you, you will be no different than his first wife. You might struggle to leave this role because you would be saying you don’t like his company, that he’s worked very hard for. Go ahead but be aware of the life you are choosing and it’s likely to be a very hard relationship.