Read what I have write. Is not recommended if he want to do it he can do that. That's on him.
I say what I did and hundreds of subq pining later didn't had any issues.
I and many other do the same. Pinning subq is not the same as pining IM.
People who are diabetics and use insulin pen or syringe do the same. They don't change the needle multiple times everyday for the next 30 years or how long they live for.
I take insulin 2 times and sometimes 3 times a day. I don't use 3 different syringe to pin that. I fill the syringe in the morning with what I need for that day and I use it all day And years later I am alive and well. Me and many other.
Is not recommended to do IM injections by yourself but here we are. Thousands of pining later didn't have any issue.
Harm isn't coming from using the same syringe to pin subq. No one got a infection by using the same insulin needle for subq pining.
For example if a wood splinter get in your finger and you manage to get it out right after the finger wil be fine. Subq is different than IM.
But if you want change that needle each time because that's what you suppose to do.