I'm going to muddy the waters even further
. We have been told that Entyvio works better for the large bowel and Stelara for the small bowel. My younger daughter has been on Entyvio infusions, but the pen is not available in the US. She has not been on Stelara but because she has both severe autoimmune arthritis and Crohn's, she's been on 11 biologics (she is currently on 2 biologics - one for her Crohn's and one for her arthritis - since her arthritis has been very, very difficult to treat and unfortunately doesn't respond to the same meds her Crohn's does).
On the forum, I have heard many parents say their kids prefer syringes because they can control the speed of the injection. My daughter (actually both my daughters - the older one only has arthritis, no Crohn's) prefer pens. They say that the injection is over much faster, they don't have to worry about making sure they're injecting at a particular angle, they don't have to see the needle and honestly that with the pens, they're so easy to use that a monkey could do it! (You literally just have to press a button).
I will say that their doctors encouraged them to start doing their own injections early - to give them a sense of control. My older daughter started at 15 and my younger one at 13. They have both used prefilled syringes, regular syringes that you have to fill yourself and pens at various times. They find pens just much more convenient. We always made it a routine - they chose the injection site, iced before the shot, did the injection while watching TV so they were distracted and if necessary iced after the shot. Dessert after the shot helped when they were younger. The click the pen made never bothered them and they've used MANY different pens between them - Humira, Simponi, Enbrel, Cosentyx, Taltz, Tremfya etc.
They have both also tried Buzzy and my younger daughter occasionally uses it for a biologic she is on for her arthritis. She thinks it helps somewhat. My husband has the same type of autoimmune arthritis that my girls have and he HAS to use Buzzy and will absolutely not give himself his injections (my daughters think this is absolutely hilarious) - I have to do them for him. He also ices for a good 15 minutes before the shot. My husband has been on several different biologics and absolutely refuses to use a pre-filled syringe - he does NOT want to see the needle.
I would say that if your son is scared and is leaning towards the pen, then that's what I'd choose.
I don't know if infusions are an option, but a LOT of kids/teens/adults with needle phobias prefer them because then someone else is in charge of the needle. My daughters didn't mind them, but found them inconvenient as they got older - our insurance generally did not approve home infusions for biologics (but it would for Solumedrol for example). I don't know if that's an option for you, but I would say don't rule it out since it sounds scary. Entyvio is a 30 minute infusion usually every 8 weeks, so it's not too bad.
But going back to injections, I do think it really helped my daughters to do their own shots - of course they were very nervous when they started with pens and then again very nervous when they had to use syringes for Methotrexate (which are not pre-filled). But now it is so normal and routine that "shot day" is not something they dread or even complain about. In fact, most of the time they look forward to shot day because it's an excuse to watch TV and eat dessert
and they know they'll feel better after it.
I will also say that my girls were able to store their injections at their colleges. When they lived in dorms, we got them mini-fridges with locks but to be honest, since they usually lived in suites and apartments, they used the communal fridge. Never had an issue. We did investigate having the college health center nurses give injections, but my daughters both decided that it was easier to do it themselves.
In terms of pre-filled syringes, my younger daughter has used Cimzia which comes in a syringe similar to Stelara. It's thick, so she would inject it slowly. But she really did much prefer pens when they were available.
My daughter Crohn's has honestly done best on anti-TNFs like Humira and Remicade, so I can't really say whether Stelara or Entyvio would work better for your son - only that we have been told Entyvio tends to work better for inflammation in the colon.