It shouldn't make a difference. When was the last time you gave him a pill? With ivermectin-based HW medicines, you can go as long as 45 days between pills without a loss of effectiveness.
In other words, they had to give the dogs 25 times the normal monthly dose before they got sick from it. So a second pill in a month shouldn't do any harm, like the vet said."A rising-dose safety study conducted in roughcoated collies manifested a clinical reaction consisting of ataxia, pyrexia and periodic recumbency in one of fourteen dogs treated with milbemycin oxime at 12.5 mg/kg (25x monthly use rate). Prior to receiving the 12.5 mg/kg dose (25X monthly use rate) on day 56 of the study, all animals had undergone an exaggerated dosing regimen consisting of 2.5 mg/kg milbemycin oxime (5X monthly use rate) on day 0, followed by 5.0 mg/kg (10X monthly use rate) on day 14 and 10.0 mg/kg (20X monthly use rate) on day 32. No adverse reactions were observed in any of the collies treated with this regimen up through the 10.0 mg/kg (20X monthly use rate) dose."