Post a 24 hours notice to enter the unit. IF he will not let you in the unit then call the local sheriff with your concern. Tell the police you are concerned that this person could be dead or seriously ill. Ask the police if they can help you out with entry into the unit.
Or the unit is major dirty...
If the police ignore you then post an eviction notice and start the eviction process.
Do you have a family members number on his application that may help you to check on him???
If the police do help you they may call the health department and close down your units due to health issues, especially the first floor tenant.
Best of Success!!!
--99.56.xx.xx