I understand he has only had his hearing aids for one year, but he is 4. If you want him to catch up, he needs to be gaining very quickly.
The language skills of four-year-olds expand rapidly. They begin communicating in complex and compound sentences, have very few pronunciation errors and expand their vocabularies daily. They can follow multi-step directions and understand explanations given for things they can see. Four-year-olds frequently initiate conversations and are less likely to change the subject of conversation to areas of personal interest. They are also getting better at sharing personal experiences without prompts from adults.
Four-year-olds are building their knowledge of written language. They want to know what words in their environment say and can recognize many letters. By the end of this year, many children understand that letters represent the sounds in spoken words and may associate some letters with their sounds. Most children also are capable of writing some legible letters and know that writing goes from left-to-right and top to bottom.
Four-year-olds have an increased capacity for learning math concepts. They use logical reasoning to solve everyday problems and can effectively use language to compare and describe objects and shapes. They can count to "ten," recognize written numerals "0" to "9" and add and subtract using numbers up to "four." Four-year-olds know some variations of a circle, square, triangle and rectangle. They know days of the week, months and the seasons, but still cannot tell time.
This are the skills he should have.
I understand that he has only been hearing for a year, but is he gaining at a speed that he is getting close to this? If not, he isn't hearing well enough, so you need to sign so he can get the language he needs.