is he on school action currently?
what are school doing at the moment?
is he 'failing' in areas of the curriculum, or noticeably falling behind his peers?
if school are providing targeted support out of their SN budget and he is responding to this, it is extremely unlikely that the LEA will consider a statement. it doesn't stop you requesting an assessment, but it is very unlikely you will get much further than a letter saying, thank you, but we believe he does not require a statement.
if he has been on the school action register for some time and getting additional support, and still failing to meet the termly or six monthly targets on his IEP, then you may be able to use this as eveidence that a statement is required (or they will bump him to school action plus and request external support). if he is meeting his IEP targets the LEA will almost certainly say no statement is required.
i don't actually know anyone with a statement for dyslexia, but i'm sure that it would be possible eventually to succeed if his needs were severe enough.
fwiw, we received 1-1 support and additional LEA funding and advice without a statement, but it is very much a postcode lottery, and is not legally binding. dd2 now has a statement in place as we move frequently and wanted the support she needs quantified as a starter for ten for a new LEA. hers is a low incidence statement for pd and as such relatively easy to argue, in your case i suspect you ought to start saving for a tribunal do speak to ipsea and the sn bods at the lea before you go any further!
best of luck!