GSD are a breed only a VERY experienced dog owner should have. They are NOT an easy dog for novices to manage. They are a high energy HERDING breed (bred to WORK) and are super intelligent (prone to problem solve or OUT THINK the owner) and can be pushy or manipulative, if not owned by an authoritative owner and confident person.
If he is not buying from a very reputable breeder (who has done TONS of health screening tests on the parent dogs) and can show PROOF of things like (testing for hip dysplasia, thyroid testing, eye testing, epilepsy testing etc., etc). for commonly problems seen in the breed (and genetically passed along) then the dog can be a VERY expensive dog to own, long term, medically speaking.
GSD are NOT the best breed for a young child and esp not for a fearful child, since they are can be fast moving (easily able to MOW a child over) and by 6 months will be minimum adult size and can end up weighing 100 lbs at adulthood.
Aside from the purchase price, most dogs and their supplies & the vet visits (in the first year) can easily cost $2,000.00 (you can google this) so you are RIGHT to be concerned about the costs. I would pay full attention to the fact he cares ZERO about your concerns, and the absurdness and inadvisability of this venture. He had NO PLAN on “how” he is going to make this work. Therefore that (in and of itself) is very predictive of the future. **DO NOT** plan to move in with him.
Did he even make SURE his landlord will *allow* dogs, 2) will allow large dogs and 3) will allow GSD (which are on the dangerous dog list) -due to reported bites and higher insurance costs? Did he even disclose to you the EXTRA pet deposit he will probably need to pay? Or is he just “HOPING” the landlord will never NOTICE a large, protective GSD? Does he have a fenced yard, or is he allowed to install one and what will THAT COST?
He will probably do this, and may “try” to make YOU the care taker (which I hope you will NOT agree to). But regardless, he live to regret it on any of a NUMBER of fronts, since I doubt his room mates will take care of the dog or supply any of the daily TRAINING, socialization and exercise needed. And if the GSD is not of an outstanding temperament… the dog may be come overly aggressive or overly protective especially, if it never gets the **needed** socialization with people and other pets, BEFORE 16 weeks. (Protective breeds of dogs can be like owning a LOADED GUN – in the home.)
This is a recipe for disaster, and I think you need to make other plans including getting rid of THIS irresponsible and brainless BF – who has not thought of anything past his own “wants” => including the safety of his son or the extra costs, – he REALLY CANNOT afford… or what happens (to the dog) if he has emergency veterinary costs he has not budgeted for, per the dog.