14 month old, diagnosed with hearing loss, can he still develop speech and be normal?

Xterrific

Hi everyone, its a long story, please bear with me.

My sister's son is now 14 months old, we used Gestational surrogacy since she had complications carrying baby and lost her twins once.

He was was born at 8 months and was under weight. But the hospital certified everything to be normal.
After 1 and half months they went for big fireworks and there were loud explosions but the child did not wake up or anything, the explosions were loud enough to startle but not enough to damage hearing, they said. they did not go to a doctor.

when he turned 2 months old, they went to canada for work with the child and started noticing the child not rsponding to sounds. he makes loud blabbering noises. at 14 months old, he has difficulty standing up. He's not interested in walking or exploring. He doesn't call 'mama' or 'dada'.

Once they were sure(when about 1 year old) they went to a doctor and he suggested he has ear wax or something in his ears and it happened because of improper positioning of head while feeding.

Strangely the hospital didn't do anything, they have to wait 2-3 months more to get an appointment and do the procedures to get the hearing back. This is strange because in India, things like this get urgent attention and the whole thing will be over in a week. So, they're coming back home next week to get medical care.
We're all really worried, will he able to lead a normal life? Can he still develop? because his actions looks like of a retarded child.

on the parenting part, i agree they were ignorant. Should have done check ups at the first sign. But they were busy moving to canada and all and the child did not get proper atttention. Also it was hard to understand that time if he's able to hear or not.

what's going to happen if he regains hearing? will he able to catch up or the damage already done?
Thank you so much for all the help.