15 months old is a growth-spurt time.
And a time of many changes developmentally. This in itself tweaks their sleep.
They do this.
Or he needs more intake. Because it is a growth-spurt time.
Its a phase.
Both my kids did that too.
Roll with it, and keep to your routine/consistency... and it will even out.
A baby does not sleep according to our sleep sense, as an adult.
Put some baby-safe toys/stuffed toys in his crib... or a Fisher Price "Ocean Wonders" crib toy in there too... we did that with both of our kids.. and they liked it and it made their crib comfy and 'fun' and they often would, before falling asleep or after waking, they would entertain themselves in the crib for awhile... before "calling" for me and crying. And as babies, my kids would turn-on the Ocean Wonders crib toy themselves to listen to the music or watch it.
And, make sure he naps during the day. This is important for overall development/immune system development/sleep.
What time does he go to bed?
I would not, change/mess up his going to bed time... because that in itself can really tweak their sleep ability. Keep his going to bed time consistent. Changing it up too much, will often back-fire.
Keeping a baby/kid up later than usual... does not work.
They will still wake. It just makes them over-tired. And over-tired kids DO NOT sleep well, nor fall asleep well, and they wake more.