I sold a house in England recently (finally after 3 years
) ... we had an offer which we accepted, then had another offer slightly higher come in.
After a LOT of soul searching we turned down the higher offer and stuck with the person who made the original offer, yeah the extra cash would have been nice - but I'd agreed a deal and to me the principle was worth more than the cash.
That being said if the legal side of things is the same as in England you'd be within your rights to accept the second offer as nothing has been signed and sealed at that stage, from the sounds of it the chap hasn't even committed to surveys etc. yet so hasn't really spent anything on the purchase which minimises the damage to him somewhat (in our case the couple involved had done surveys etc. and committed some funds to the purchase which did influence my decision somewhat, I'd have felt a cad if I'd left them in the lurch after that) ... at the end of the day only you know your situation fully, talk to your family and make the call you feel is right.