My eldest was diagnosed with autism age 5 and ADHD age 12, they tend to overlap so it’s worth looking at autism advice online as well (not saying that your son definitely has autism, just that a lot of the advice can also be useful!)
It’s hard. Really really hard. Get whatever support you can, there are usually local groups that are excellent at signposting you to the help you need. Having said that, the available support can be really limited.
AVOID ASPARTAME. Can’t emphasise this strongly enough, it tips ADHD kids over the edge. Diet and supplements can help, fish oil is useful – do some research online.
He will need much more supervision that a child without ADHD (no messing, you can’t turn your back for a minute!). Very clear rules and boundaries, having visual reminders, timers etc. Make sure he has access to fidget toys, and activities that will allow him to let off steam. Sensory play is really useful, things like playdough (learn to cook it up yourself to save money), bubbles, slime, sandbox. A large plastic container with uncooked dry rice (you can colour some of it with food dye as well) that he can drive toy trucks through.
It can take a long time to get a diagnosis, so don’t wait until then, put strategies in place now – they won’t do any harm if he turns out not to have ADHD! There are lots of websites, forums etc where you can get ideas and advice, even Pinterest – see what appeals and would work for you both.