I'm sorry to hear of your dilemma.
Giving your boyfriend 51% of your business would be a bad business decision. You state that there was no agreement with regard to your obligation to repay the seed money for your business.
While you qualify the money as a loan, and he has the right to ask you to repay the loan, he is not legally entitled to a managing share of your business.
If he wants to turn things around for your business he can do that without becoming the owner of the business.
While you don't say what a type of business you have, or how much he put in at the beginning, you could be giving him a huge financial advantage. If he loaned you $2,000 for a business that 5 years from now nets $100,000 a year, that's not fair to you.
You might offer him a share of the profits over a certain period of time to the extent that he makes changes to your business that improve the profitability of the business. Short of that, you are better off just repaying the loan.
I wish you well.
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