The most crucial thing for guaranteeing support at school would be an EHCP (the new version of what used to be called Statements of SEN). This will involve a round of assessments, typically from Ed Psych and Speech & Language therapist, possibly from OT too. The whole process can take a few months from applying to being issued the final version (if you are indeed given one - if not, that's a whole other story!).
Yes, arrange a meeting with the SENCO. Hopefully school will support you on applying for the EHCP - you can do it yourself, or school can apply for you, but imo it's better for you to do it yourself as a) you know it's been done, and b) you can appeal if they refuse to assess whereas school have to wait six months to appeal (at least, that was the case with the old Statements, may need to check this still applies for ECHP).
In the meantime, try and pin down with school what support they can provide. Try and get a clear list in your mind about what you think he needs at school: support in classroom? specific support in learning? social support? language? OT? classroom breaks? etc etc.
They will have funds to provide some support even without an EHCP. He should be put on the SEN register at school, and on School Action/School Action Plus, which is a less legally binding form of support than EHCP, but is halfway there if you have a decent school.
Also look at local NAS support groups to meet local parents, that can be great for emotional support and network, just to meet parents in similar positions.
Look at other therapies that may help at home - have a browse through these boards for loads of ideas/books etc.
Take care, and I hope you have a supportive family/friends too. All the best.