Are there other difficulties mentioned?
Receptive Language is understanding of spoken language.
Expressive Language is talking (vocabulary, grammar, sentence structure, narrative).
It would be unusual to have an difficulty with understanding without having a difficulty with talking too (understanding comes before talking).
It may be that his attention levels are impacting his ability to understand and process language, which is why they might be looking into other things.
Language disorder can be a diagnosis by itself (in which case it it called Developmental Language Disorder) or it can be associated with other conditions (Autism, ADHD, learning difficulties...)
Autism would include differences in his social skills, inferencing, ability to understand and respond in social situations.
Have a look at RADLD which has some info on longer term outcomes, support needed etc.