It could be a case that the owner has miss worded what he was saying to you. To the best of my knowledge there are 2 ways that a property goes under offer.
1 buyer viwes propert likes it makes formal offer property is formally under offer
2 seller gets a few notices of interest goes to closing and picks the best offer (not neciserily the most moeny but more often than not) then the property goes formally under offer.
By the sounds of it you would be worth while giving your solicitor a quick call to find out if the property is formally under offer to understand where you stand when you go to view it tonight.
As druidh mentions you can get your solicitor to stick in a notice of interest which should allow you to bid on the property or at worst you you will be notified if it is under offer/off the market.