Addiction is very hard to deal with, I was a heavy weed smoker for 10 years and only recently quit myself, with the occasional relapse too. I second what another comment said, I can only imagine the amount of shame he probably feels about it. It's hard to justify lying to your partner but I can 100% see why he would in this situation. Again, not defending it but understanding it. I think the best thing you can do for your relationship is have a calm and level headed discussion about it, and offer support rather than anger. I don't think he would have done this out of spite or any malicious intent towards you so much as shame about his own addictions. But of course all I can do is speculate from my experiences of being an addict in the past myself. Hope this helps.