If you end up serving a eviction notice on this issue, which I can almost guarantee you will end up having to, you can not win the eviction outright. Initially the first eviction will be voided by him not smoking for the allotted time period. He will then smoke again and the second eviction notice (within 6 months) will result in a board hearing.
You must have another tenant at the hearing to testify in person, written statement from tenants is not sufficient they must testify in person, you will likely have the tenant be given a conditional stay if they guarantee they will not smoke in the unit again. Ask that you be given the right to immediately apply for eviction of they break that agreement. That will of course be denied as the board will want the tenant to have the right to another hearing.
When they smoke in the house again the next eviction hearing, again with a tenant witness, should succeed in eviction.
Good luck. This will be a very long process. You might hasten his departure if You schedule regular inspections of the unit. He may also be growing.