So OP you save - he spends money on his business. & on top of spending money on his business, he also needs money from you to support his business. Outgoings are low you say - and he also has a job. So I don't quite understand why you need to be paying for his business in this way, at all
Also what "Can'tChoose" said:
I share PP concern that a single delayed pay check has left him that short.
If you earn the same and split costs equally how have you ended up with more than 3k savings while he is £500 short? Does he spend the rest of his money irresponsibly? Or he he spent a similar amount of his own savings on his business too? If so, 5K+ on a hobby when he clearly isn't flush seems bonkers!!
Whilst I agree that finances between a couple should be shared, I don't believe that one person pulling on the financial resources more than the other, does actually amount to sharing. Thats not equal, thats an imbalance. I also don't believe that those who say "you must share" would have been willing to tie their finances in with a partner, unless there was a clear indication that he was financially responsible and trustworthy. To me it would be the same whether living with my partner, or in a flatshare. If we are running a home together then we do that equally.
As it is, he has borrowed from you to start his business, now he wants to borrow from you to get through the month. I don't think you've seen the last of his borrowing from you. What if you didn't have the money, I wonder? Would he have done what many have had to do - gone out there sourced whatever business advice, funding and grants are available, spoken to his bank, sorted overdraft etc? Do you know what his finances look like?
Just looking at this practically really. In your shoes I wouldn't lend him the money, but you do say he is normally good with money and you trust him in which case, you may feel fine to do so really, you are just wondering about a few things. However - you say this business, which he has thrown money at and also had money from you towards it, could just be a "fun" business if he doesn't make any money from it, then thats ok. If its just for fun, why the big financial outlay, + leading to him being too skint to get through the month?
I wouldn't say Im brilliant with money. But missing just 1 paycheck wouldn't leave me having to borrow £500, although it truly would be a month of scrimping. Especially if I were living with a working partner, and our joint outgoings were low anyway.
If you're giving him money again try to get a concrete payment schedule from him. Don't rely on him setting up a standing order or direct debit - after all if the time comes again that he is short, he will be borrowing from you in order to pay back the money he owes you...