My 11 week old son goes from wide awake to overtired and cying, there's nothing in between....?

I have tried putting him down for naps at various times but he just stays awake and then cries because he gets overtired. I really don't know what to do, i've tried a mobile, dimming the lights, keeping quiet, rocking him but he just fights sleep. Has anyone got any suggestions on how i can help him go to sleep?? I've tried leaving him but he works himself up into a state and it's impossible to console him. It isn't colic either or wind as i put him down long after he's had a feed when he definately isn't hungry. Any tips would be great. Thanks for your comments.