It's great that he has been honest (although he should have come clean before getting into a relationship).
Respectfully, you don't know him well enough to determine the severity of his addiction and if he's been making steps to have a better quality of life. You don't know him well enough to determine if he has his life sorted out.
We all have our problems and areas in life which could use improvement...respectfully maybe you should end things, and find someone else without a substance abuse problem.
This will be a lifelong battle for him, there will be relapses, there will be stagnacy, there will be times when the addiction will be in control, and he will possibly do things to feed his addiction, etc...
This is a lifelong battle, with no full recovery. He will always be fighting these urges.
Of course it is possible for anyone to become an addict at any stage of life...
He has already become one.
If you have limited to none experience nor exposure of dealing/struggling with or dealing with someone with this problem, it may be best to end things. Your expectations and optimism will often get crushed.
If you proceed, don't have any children together, don't move in together, don't marry him.