All of the sleep schools I know of advocate crying it out or controlled crying of some kind & I won't do it. Just my preference, so sleep school isn't an option for us.

I need some way for him to realise I am still there during the night when he wakes up without him having to see me.

Lisa, other than separation anxiety, I was thinking it might have to do with having a day sleep & maybe he's just getting too much in the day. Is your son still having a day sleep? Zander's is generally only for an hour though so it's not like he's sleeping too long.