To be fair, depending on how much training our man has been through, and what he's been training, together with how good his diet is, there is a reasonable chance he could get 6kgs packed on in a year.
I managed it in 4 months, but that was because:
a) I had never really done "proper" leg work before (squats, deads and more squats);
b) I sorted my diet out.
I'm not saying it was easy, in fact for most of it I was in a permanent state of DOMS, but it is possible, assuming we're starting with somebody completely green here. Most of the weight (as you can/will see from my "first to 90kgs" thead) was not on the top, but on (literally) the bottom: I've had to stop wearing half my wardrom because my thighs and bum is too big now. Get a bit of blood perfusion going on there, and his weight will pile on... assuming he's not already achieved those easy gains.
The qeustions I posed to him were to get an idea of where he was - chances are Joe Average has never thought about proper leg work before, and has never really thought about his/her diet when it comes to gaining/losing weight properly.
I'd set about doing big compounds, with weights as big as you can manage for 5x5/whatever. As FF has often posted: eat like a MF, train like a MF, sleep like you've just been a MF. I might be paraphrasing slightly, but that's the general just of it.
Get that sorted, and you'll be awesome.