I would encourage him to see a doctor and get a checkup.
If he has nothing to hide and loves you like he claims, tell him to see a physician - go with him to ensure that he does go.
It could be due to the pot, it could be he is taking medications of some type that inhibit the libido, or he might have 'caught' something else.. Who knows. But if he does love you he will do what is necessary to reassure you.
Either way, wouldn't you want to know the reason? If he is lying on all fronts, then this will help you determine what YOU will do with your future.
Good luck, dear, and keep us posted.
http://smileys.smileycentral.com/cat/15/15_9_16.gifIt's better to know than to be in constant self-doubt.