As SpursMum said, there are other people on here who are absolute mines of information and I'm sure they'll be along soon. My son was only dx three weeks ago (after a year of us suspecting) so I'm relatively new to all this and still in the process of finding out for myself. The single most useful source of information for me has been this website! There really is nothing like the experience of other parents, especially with something as wide ranging as autism. I always knew that the spectrum was huge, but it's only since coming on here that I've realised just how different and individual each child with ASD is. My son is younger than yours (3.5) and only just becoming verbal so we are in very different positions.
Nevertheless, I'll tell you what we've found most helpful so far:
National Autistic Society. Their website is hugely informative and you can ring a helpline too. There are local branches you can join and meet with other parents. The website has hundreds of links so you can research almost every aspect of ASD and many interventions and therapies.
BIBIC (British Institute For Brain Injured Children) in Somerset. They would do a holistic assessment of your son and give you an individual programme designed to help him achieve his potential (this might include sensory exercises, activities to promote learning, activities to improve fine and gross motor skills, to manage behaviour etc.) You have to pay for this, but there is funding available. The first step is a telephone interview which is free and doesn't tie you to anything - they take a detailed history for about an hour and then tell you the sorts of things they'd be able to offer. You then decide whether to carry on. It's definitely helped my son to make progress, but don't know how useful it would be for an older, more able child. If he has sensory difficulties then I would say give it a try.
Although my son is doing well on the BIBIC programme we are looking at other methods of helping him as well - and again, this site is brilliant if you want views and advice on different methods of intervention.
Will do another post with links to the above - can't remember web addresses off top of my head!
I think the feeling of confusion and 'swimming in the dark' is the one I found most difficult when we first suspected our son had ASD. At the beginning it's easy to be swamped with so much info that you can't take it all in. It does get better- I'm no expert, but I feel hugely more well informed than I did six months ago and more in control as a result. The mums on the SN board are a very supportive bunch and really take time to answer the most basic of questions. So you've found the right place!