Asthma's a tricky thing, but here goes! Can i just stress that I'm not an asthma nurse or a doctor, but I've had asthma all my life and for portions of it it's been brittle, so it's something I've had to learn an awful lot about.
I don't want to scare you, but you're absolutely right to be worried because asthma is a scary thing. It's manageable though, and the more you get to know your son's asthma, the less it'll worry you.
The cold is probably what caused the flare up, what usually happens is that his medication won't get down well enough due to his airways being affected by the increased mucus from his cold, so it doesn't work to its best ability and that's when problems can happen. Personally, and this is just me, I always double up my steroid inhaler ever morning and night if I've got a cold, but that's something my doc and I have discussed on many occasions before. It's not uncommon for doctors to ask you to double-up your preventer when you have a cold, so it's best to have more than one in the house. Some surgeries don't like you to do this - I've found insisting really helps! ;)
His peak flow might not be something that needs doing every day, it's usually just something that is checked when you go to the asthma nurse once every 6 months. That's routine, and it should always be done at an asthma check up. It checks lung capacity/strength and is age/height-related, for example mine should be around 470. If it dips further than 85 points below this, I know I'm going to have problems with my asthma. At his age, the doc or asthma nurse will be able to tell you what his peak flow should be, they have children's ones with scales that only go up to about 350 for littlies.
At night, prop him up on pillows. It's best not to lie flat, and certainly if he's been rushed to A&E they won't have let him lie down. It's much harder to breathe when lying flat, so prop him up a bit on pillows so if he does cough, it's draining away instead of straight back down.
Whenever he gets a cold, he's more prone to a chest infection, because he's asthmatic. It'll take him longer to get over colds than other people, because they will affect his airways more. As a result, the only way to prevent this happening in future is to speak to your doc about what he thinks you should do at the first signs of a cold. As I said, my doc always had me double-up my inhaler morning and night, so I was taking 4 doses instead of 2, but you must must must speak to the doc for him to check that doesn't take your boy over his daily allowed amount (not sure what preventer he's on, but as an example you can only take 1600micrograms of becotide a day, so the doc would need to check dosages and make sure it was safe to double them) Whenever he gets a cold he's also more likely to need his reliever inhaler more often too. These aren't things to worry you, as long as he's got enough medication at all times.
Preventing asthma attacks is all about knowing triggers, and a lot of that is trial and error, but colds are usually one!
I hope that helps a bit, let me know if you've got any more questions and I'll do what I can! x x x
Em x x x