Well, IME, if they are talking to you about a language unit, then he must have some fairly obvious SEN.
In my LA, there were 70 children put forward for 6 places, and they were only the ones who met the criteria - which included there being no behaviour problems, except those secondary to the SLI and expected to settle down once they were in the right environment. Children with major behaviour or attention problems, thought not likely to benefit from the intensive provision on offer, weren't even considered! The idea behind language units in my LA is that children get 2 years of intensive remediation, and then they can be returned to mainstream.
Many parents were never told what a language unit is, never mind where they are; or that their child needed a language unit. Speech therapy reports were written, IMO in such a way that parents did not understand the significance of them, such as:
"On CELF-3 UK, X has a standard score of 65 on receptive language....."
Whereas the professionals knew that this meant X had a severe receptive language disorder."
Many parents are put off language units, because they tend to be in primary schools in more deprived areas. Its just a fact that only under-subscribed schools are going to have room for them; and SEN children are not going to stick out like a sore thumb in them, because some of the other children will have language delays, etc for different reasons.
I sent my DD with a receptive and expressive language disorder at 4 to a language unit, 10 miles away in another town in a deprived area, because I was told that she barely understood a word anybody said without visual clues and she would only cope in mainstream for another term or two! Our local primary was outstanding in the league tables; but not outstanding at SEN!
Does your child has a verbal dyspraxia? Read the article by Pam Williams, Chief Speech Therapist at the Nuffield on verbal dyspraxia on the Dyspraxia Foundation's website, if so. In a language unit, the speech therapist can deliver say 10 minutes of speech therapy to your child three times a day, or whatever they think is required.
If your child has a language disorder, the pace of learning can be tailored much more to the individual, than in mainstream. A language unit is used to say doing division; and then having to go back to it the next half term, because the children struggle with concepts, mathematical language, attention, etc; whereas in mainstream the pace of the curriculum does not allow for that - if children miss a key concept, tough!
IMO, there is no point having a split placement for a language unit. If you want mainstream, then you might as well just get as much support in class and direct speech therapy as you can. As a friend of mine, a top class speech therapist said on this subject:
"Why give a child a crutch and then take it away some of the time?"
Either a child needs to be in a class of 10, with a high staff ratio, intensive speech therapy, a specialist teacher, etc or they don't!
If they are talking to you about a language unit , I suggest you give it some serious thought - if your child's problems are remediated, they can always return to your local primary. Try ringing the Afasic Helpline, where you can discuss this question further.