Although it won't be very accurate you can get a general idea of age by weight. Measure how heavy your puppy is in Kg and work it out using the table on the webpage below. Much as you won't get a definitive week of bith at least you could get a sense of whether or not the guy you got him off of was telling the truth which, due to the teeth thing I doubt somewhat (though I am by no means certain - teeth are lost at different times depending on the breed, and I'm no expert on cocker spaniels specifically).
http://jn.nutrition.org/cgi/reprint/134/8/2027 S.pdf
Good luck, I hope you get a satisfactory answer.