If he's trying to gain weight relatively quickly he's probably not gonna wanna do straight strength training, particularly if he's not an experienced lifter.
Eat more than you currently do, do strength work, but make sure to emphasise your accessories in order to actually build muscle.
The purpose of strength training is to strengthen muscle you already have, if you're trying to get bigger - you need to focus on that first.
You will get stronger as you get bigger, but your focus should be on size initially, then once you're at your target you can improve the strength of the new muscle.
If you eat enough, and depending on your genetics, you could easily put on 10kg in 6-12 months.
I'f you're on a budget, forget GOMAD, get Jersey milk from either Tesco or Sainsbury's.
It's delicious, and far more densely nutritious than regular full fat milk, meaning you don't have to drink as much of it.
For cardio/stamina - find a circuit that takes you 10-20 minutes (or even a Tabata) and do it now, record how fast/how many/whatever... you do it.
Have that as your benchmark, do it weekly, make sure you don't fall too far behind what you could do at the beginning.