Job one is getting him to use headphones; all of the usual things that entertain littles tend to be a bit noisy, and noise bothers passengers no end (especially on flights that are crossing time zones, as folks tend to like to nap more in those circumstances.) I found that a good way to get babies to deal with headphones is to wear them in their presence, and make a big deal out of putting them on, etc. Their curiosity will get to them and they will want to try. Since he likes to throw, make sure that any electronic screen you give him is in a thick rubber bumper case and has a strong screen protector applied; one good slam against an armrest and that iPad could be toast. As to what to watch, try downloading YouTube videos of babies. Babies love watching other babies (including the animal kind), so that might hold his attention better than an actual "story" video.
Have one of his parents record the board books you are bringing, preferably the parent who usually does bedtime stories. That way he gets a read-along that won't bother other passengers, and that *may* actually help him sleep.
Play-Doh is NOT a good idea, at least not the real name-brand stuff, for 3 reasons: 1) it stinks. Other passengers are likely to have issues with the odor, 2) It crumbles easily and will cling when it does; very difficult to clean up. 3) It leaves an oily residue, and this is especially a problem if it crumbles on a seat; many a business suit has been ruined by sitting in Play-Doh residue. If you must use a "dough" product, please go with Crayola Model Magic, which is odorless and does not contain any oil.
Be very careful with the "ball on a string" idea; the FA's may tell you it is a hazard. Make sure that the ball is soft enough not to make a thumping noise, and be sure that the cord is no more than a foot or so long; having it fly backward over the seat-back into another passenger's face or drink would not be a happy thing. Also, as you are sitting in a bulkhead seat, you will have trouble stashing his stuff if the fasten-seatbelt light is on. I recommend bringing along a carseat under-mat that has pockets in the front; you can put it down on the seat underneath you and use the hanging pockets to stash small items in flight. If you don't already have one, something like this would work:
https://www.amazon.com/dp/B075K7JQJD/ref=sspa_dk_detail_3?psc=1 (Or it would be easy to make one if you sew.)
Also, I know it's too late for your flight, but for others reading the thread: 15-24 mos is PRIME squirm time for toddlers, the phase at which holding them on a lap for a long flight is pure hell on the traveling adult. If at all possible, buy a seat and put the child's carseat in it; it will keep him safely contained so that adults can get some rest.
One other thing: as you don't know whether or not this child is prone to airsickness, best to travel prepared. Don't feed any dairy within 2 hours of boarding the plane, and don't feed any during the flight. (Note that while dairy does not CAUSE airsickness, the smell of it and the difficulty of cleaning it up will make dealing with an airsick child that much worse if it happens.) Stick to clear liquids that are less likely to stain. Carry your own toddler-friendly snacks; mine always liked Nilla Wafers, which are not very messy as cookies go. Bring a white cloth in case you need to scrub up clothes or the seat; wipes don't work well for that. (I always carried a couple of flat cloth diapers in my bag for this purpose; they are absorbent and work very well for scrubbing, and you can put them across your lap to protect your clothes from crumbs, etc.) Also, young kids usually can't use airsick bags properly, so when ours were little we carried along a small beach pail for this purpose.
