Difficulties with language is not actually a symptom of autism. Autism Spectrum Disorder involves difficulties in social communication and interaction, as well as restricted and repetitive interests or behaviours (DSM-V, 2013). The term "social communication" is referring to difficulties in the social aspects of language and other communication, such as matching verbal and non-verbal behaviour, rather than difficulties in language.
The language difficulties as described in the question sound like they could be related to the age-equivalence of the individual, although of course physiological hearing difficulties can't be ruled out until it is assessed. All children progress through a sequence of language development (Tager-Flusberg et al., 2010):
- Babbling, gestures
- Typically developing children aged 6-12 months
- Non-imitated, spontaneous single words
- Some of the speech is intelligible, and uses consonant sounds typical in babble
- Typically developing children aged 12-18 months
- 2 and 3 word combinations, including nouns, adjectives and verbs
- Typically developing children aged 18-30 months
- Children create sentences, now using plurals and prepositions
- Typically developing children aged 30-28 months
- Language for a range of topics, using complex grammar, in different discourse contexts
- Pre-school aged typially developing children
Tager-Flusberg et al. (2010) have a great table (Table 1) in their article outlining their proposed benchmarks for each of these phases. As the child progresses, their phonology (speech sounds) improves as well as their vocabulary, grammar, etc. Thus, your brother may be imitating the sounds he hears as best he can within his level of language development.