There is the EYFS assessment framework, if you google for it you'll find what kind of expectations your child has been assessed against. That may help you understand the 'speaking' result. Though it is also possible that the teacher made a mistake. Mistakes do happen. It is not very important however! No one will ever ask 'did he achieve expected at EYFS?' ...
To me, it suggests he's not doing very well at all.
Well, he has indeed not achieved GLD ('good level of development' I think it is) - that is defined as at least ten 'expected', some of which have to be specific ones. So he has not achieved the level of development the government 'expects' by end of reception. However: He is in good company. There is very good reason to argue that it is the expectations that are too high, rather than your son who is not doing well enough.
If you are into statistics, look up the documents at this [https://www.gov.uk/government/statistics/early-years-foundation-stage-profile-results-2015-to-2016 link]: It is an analysis of the EYFS outcomes last year. It shows that only 62% of all boys - not even two out of three - achieved a GLD.
And only 59% of summer-borns achieved a GLD. Assuming that the girls/boys ratio of achieving GLD is the same within the summer-born children as overall, it is likely that only about 1/2 of summer-born boys achieved a GLD. Add to that the fact that 'summer-born' technically includes 5 months of births, April-August (nearly half the year!), and it is likely that the majority of those summer-born boys who DID achieve GLD, are the 'older' ones - April/May birthdays, rather than July/August birthdays.
The fact that one out of two summer born boys did not achieve 'GLD' doesn't imply that half of all summer born boys are behind where they should be, but that they are being measured against unrealistic expectations.
BTW this is a significant improvement compared to earlier years, where far fewer than half of all summer born boys achieved GLD.
That is probably why your DS' teacher is not concerned. Your DS' only 'problem' is most likely that he is still very young, and as a boy a bit slower in developing than girls (on average). There is nothing you can do about that! And there is nothing 'wrong' with him.
What would you suggest I could do to help him catch up?
Well there is nothing you can do to make him grow older faster than his classmates!
At this stage and age, so much of it all is about 'development' rather than any particular 'teaching'. It happens all by itself (as in, there is nothing particular you need to do about it).
Although, what happens sometimes is that children learn at age 4 that they are not 'as good' as their friends, that learning is hard, that reading is 'not for them', that school is essentially pointless as it goes over their heads, that their parents are disappointed in them, ... and that is awful baggage to carry around, and might stop a child from catching up once their level of development would generally allow them to.
So in sum, I would be very careful with any attempts of pushing your DS. What he may need most of all right now could be a 'break' from constantly feeling 'sub-par' (in some schools/some reception classes, there is a lot of comparing going on...). However if he is happy and willing, you could simply make sure he has plenty of opportunities to enjoy stories with you, you can keep up a little routine of daily short bursts of reading practice, you can incorporate numbers into your everyday activities. Perhaps suggest for him to write a postcard to his friend from holidays? Things like that.