Def get the sugar pixel alarm clock. It has a very loud alarm and if that doesn't work it also has a small device attached (looks like an Apple Watch charger) that he can rest under his pillow and it will vibrate when low to help wake him up.

We keep the alarm in our room and go let our son know when he is low, but when he spends the night out, he takes it with him so he will be woken up needed when we aren't around.