It is not recommended to prefill syringes because they have a lubricant coating that degrades the insulin. I looks like you are using Lantus, there is some info on prefilling syringes with Lantus on this article, pg3 http://care.diabetesjournals.org/conten ... 5.full.pdf
You can make him a dummy syringe with colored fluid in it......
that way he can compare the two.... and not accidentally put too much insulin in the real one.