It's a limited company (presumably from the language used anyway) so he isn't going to be liable for any debts unless he's given a personal guarantee. Ignore that piece of advice from ubasses, it's irrelevant.
Likewise madmaggot and ubasses, he's not asking for money for his shares so valuation thereof is also irrelevant. He's gifting the company to Andy but wants a repayment of the £2,500 he's loaned the company. Andy says the company can't afford that.
If the business can't afford it Andy then there are fundamentally three (or four) options:
1 - Obtain an overdraft or loan to pay it off.
2 - Agree to pay him up over a period of time you believe you can afford (£200 per month or whatever).
3 - Put in money yourself to replace the £2,500 loan to him and repay from that.
4 - Find a third party who wants to invest in the business either for a share or an interest return and obtain funds from them to repay it.
That's pretty much your options. If you can't get an overdraft, don't have the funds yourself, don't have a third party (or don't want one) then his options are to accept a payment plan or close the company down, liquidate the assets and get what he can from it. As a 30% shareholder there isn't an awful lot you can do to prevent him refusing the payment plan if he so wishes. He could also sell his shares directly to a thirs party but there may be a shareholder agreement providing you with a right to veto prospective new partners?