@francohoops Hi, I'm a respiratory specialist nurse and asthma is one of my main areas.
The Symbicort Turbohaler 100/6 can be used as a preventer and reliever (so your sons old brown and blue together). It's important to note at a "Turbohaler" is the type of inhaler device, and doesn't suggest which drugs are in it, whereas the "Symbicort" is the drug information. I only mention this because other people may be on a "Turbohaler" but it may contain different drugs to the Symbicort.
I suspect your GP has suggested starting this for a number of reasons. The guidelines identify that patients who have had a hospital admission in the last 12 months should be stepped up, and also because it can be used as a maintenance and reliever inhaler (MART like a pp mentioned), it can actually therefore be used in quite a low dose of inhaled steroid.
The "100" is the steroid and the "6" is the reliever.
In someone with asthma, the steroid part of inhalers should never be stopped / weaned down / eased off totally. Only under very certain circumstances. So I suspect that your GP meant to wean down to the minimal dose possible. Starting at 2puffs twice a day is the equivalent steroid dose to what your sons brown inhaler would have been so it's reasonable to start here to get used to the new inhaler, and then it can be reduced to one puff twice a day if he doesn't have any symptoms at all - but usually step down after 3 months.
If you son has symptoms during the day, he can take an extra puff as required. Maximum of 8 in a day.
As previously pp has mentioned, make sure he rinses his mouth out.
Depending on the type of brown inhaler he had before (if it was a puff one?) this may be a different technique for him as it's a dry powder, so please make sure you and he watch a Symbicort Turbohaler video on asthma UK or RightBreathe to make sure the correct technique.
Hope that all make sense, please let me know if you want any more info.