As long as he's in the house, he'll still put out co2 that will get sucked into your grow room, then into the tent. Your breath also has a significant amount of co2, more than the dog. A co2 meter would be handy, but they are really expensive. They sell a giant syringe with "gas detection tubes" that is cheaper, but the tubes are $5.00 a piece and can only be used once. As long as your place has a good air exchange rate, you should be fine. Normally, the outside air is 300-400 ppm, in urban (polluted) areas, it will be higher. Below 200 ppm, your plants will stop growing. You should be fine, as far as co2 levels go.