The first thing you have to do is see your current head teacher and discuss future schools for your child. Then you have to work towards their entrance requirements.
You need to know what the entrance tests for the 11-18 schools entail.
Do they use the ISEB 11+ papers (aka girls' common entrance), their own similar papers, verbal/non-verbal reasoning etc.? It is really important for you and the prep school to know.
If it is verbal/non-verbal reasoning, he will be being prepared for this already (as much as you can) because this is the type of testing that senior schools use as pretests for 13+.
If they use the ISEB papers, then your child should be able to tackle these, as they are very similar to the 13+ format, with the 13+ a natural progression (it is expected that in co-ed schools, that pupils will cover the 11+ work before tagging on the 13+ material).
If they set their own papers, it is helpful to get a look at past papers. If they don't provide these, then it probably isn't an important part of the assessment process for that school. It will certainly be based on the work he is already doing.
If he is struggling with maths and English, he is unlikely to be a scholarship candidate. Again, if you want details of scholarships, you have to get that from the individual schools.
If you seriously think he is not being taught properly and not reaching his potential, then you should move him. If he is simply not a high-flier academically, then keep him where he is.
The key thing is to communicate with your head ASAP. It's what they are there for.