As a landlord-tenant attorney and property manager, i can tell you that at least taking one legal step forward may help solve your problems. You have been to easygoing with this tenant. You must let him know that 1) you will be charging late fees each month from now on (make sure that's in your lease), and I suggest you provide him with a rent statement that shows what's due and what's been paid, and 2) that if he is late one more time you will serve him with a rent demand.
Many times, that will be enough to get somebody back on track. you have to let them know from the start that as long as you are providing a habitable apartment, the rent must be paid in full and on time.
Serving a notice terminating his tenancy will mean that once you serve that notice you can no longer accept rent from him without vitiating the notice and that you will have to take him to court to evict him. That should be your last recourse. Good luck...
Tony L. D'Anzica, Esq