[Help] 3mo. puppy left outside all day (with another dog)

I just got a German Shepherd/Lab puppy a few weeks ago and hes been great with training. I took some time off work to get him comfortable at home with our other dog (9yr old Cattle Dog mix) I feel like hes learning things really fast, doesnt chew random things, waits at the door for me to let him out so he can pee/poop, already learning to sit, Ive hand fed him a lot so hes gentle, and hes great at playing with the older dog. For 3 months, hes pretty big and seems to be picking things up quickly. I work full time now, but my sister is at home with him most of the day, leaving him in the crate when she goes out to run errands. The thing is, my sister is about to start a full time job in a couple weeks and wont be able to watch him all day. My question is, should I be concerned with leaving him outside most of the day with our other dog? They get along just fine, and he mostly follows her around all day, I've watched him from a distance when hes in the backyard and he seems fine, I honestly feel ok with leaving him out there most of the day but still need the advice. The backyard is completely fenced off, the dogs dont bark, its a good neighborhood, we have a pool but its fenced off... so what should I be concerned about?