A Sydney toddler had only just learnt to walk when his mother's boyfriend allegedly attacked him in February.

Now the boy, who was 13 months old at the time, will never walk again, police say.

The toddler left in a wheelchair for life after allegedly being assaulted by his mother's partner.

On Wednesday, a man, 25, charged with the assault, appeared in court for the first time and was formally refused bail.

A woman, who said she was the child's godmother, said the severe brain and spinal injuries caused by the vicious assault had left the toddler in hospital for at least two months.