AS little chronology would be useful here.
When he was in primary, what was his behaviour like? And how was his attainment - e.g. SATs results - at the end of KS1 and KS2?
If he didn't reach age related expectations in reading at primary, what did the school do to help, and how did you support that at home (daily reading to an adult is a normally-accepted aspect of homework for all primary pupils, up to at least mid-KS2)?
If there was a pre-existing issue with learning or behaviour in primary, what communication was there between primary and secondary? Is your DS on the SEN list at his secondary, and was he at primary?
the thing is, this can work 2 ways round. A child who has very poor behaviour will tend to progress more slowly than their peers, simply because they spend less time listening to the teacher and focusing hard on their work, and may in more extreme cases spend time outside the classroom in order to manage behaviour. As behaviour may also be poor at home, the parent spends less time than average (or may omit altogether) doing the daily homework tasks - reading, times tables etc - as they spend more time managing behaviour, and thus a key opportunity to reinforce school learning at home is lost.
That would be 'behavioural needs having an impact on learning' - and so for the school, sorting the behaviour would be the priority.
Or it could happen the other way round. A compliant, well-behaved child finds learning very hard, much harder than their peers. This makes them feel upset and frustrated, and they may (depending on age) feel embarrassed or want to hide it. Help offered by school may make it worse, by emphasizing 'difference', or it may be that a child's learning needs are missed. The child starts to misbehave, and the frustration and anger means that the learning difficulty isn't addressed, either because the behaviour makes this very hard to do effectively (e.g. extra work at home) or because it hasn't been recognised.
This would be 'learning difficulty having an impact on behaviour'.
Understanding the early chronology of this - previously compliant until school work became 'hard', or always showing challenging behaviour, for example, alongside attainment at different ages - would indicate 'which came first' and might suggest the right place to intervene to stop the current spiral.