Testing:
1) Always test before giving insulin to make sure it is safe.
For now, your no shot level is 200 mg/dL on a human glucometer (230 for pet-specific); this will lower as you collect data around the middle of the cycle to know how low he is going.
2) Mid-cycle tests between shots - whenever possible, test around the nadir (lowest glucose level between shots) for your insulin, to see how low he's going. For Lantus, this often falls between +5 to +7 hours after the shot. Some folks do this on weekends or set a clock for the middle of the night to get this test done as it helps determine dose adjustments. This number should be at or above 50 mg/dL on a human glucometer (68 mg/dL for pet-specific meter).
3) a before bed test is helpful in determining if you need to break out some higher carb food and steer the glucose level or go to bed with some peace of mind. Steering means giving 1-2 teaspoons of high carb gravy, waiting 30 minutes, and re-testing (repeating as needed) to make sure the glucose stays above 50 mg/dL.