Controlled crying OK for 20-month old?

[choucroutegarnie]

Our 20-month-old son has been a fab sleeper since 10 weeks - 7pm to 7am, that kind of thing. Never a problem falling asleep on his own.
Now he's hysterical when I leave the room - never with his dad, which is weird. After 5 minutes I will go in to calm him down and reassure him (not pick him up) but it seems to take many attempts before he finally settles. It feels very cruel now - he's actually howling as I write this - and I could do with some advice!
Thanks

Is his dad not available every night?

How long does it take him to fall asleep if you stay?

Is he teething? My DD is going through phases of this at the moment. XX

Thanks for the quick responses! He seems to have settled now, after 2 rounds of 10 minutes of crying...
Unfortunately his dad is not around every night. Though I'd happily 'delegate' the putting to bed to him, it seems a bit of a cop-out not to be able to do it myself.
He has been teething a bit lately, but he's so cheerful throughout the day, it is quite shocking to see his mood change so abruptly. One minute, when I put him in his sleeping bag, he giggles, then the next, when I say 'night night' he goes wild!
As for how long it woudl take him to fall asleep if I stay, I am not sure but VERY LONG. Before I started this controlled crying, I stayed next to him stroking his head and he was just delighted, chatting, pointing, playing. I remember it took an hour once, and still no sign of sleep!