First off do you know what size he is hun?? Don't go on the average sizing guides whatever you do,according to them my lad is a 5'6",he is actually a 6'3"
I have a similar type build to Arnie as you know and I have found the amigo rugs to be a good fit around the chest and shoulder area,they are also nylon lined which I find better for wide types as move with them rather than pulling back over their broad shoulders,and flat wither.Saxon are also a reasonable fit.
The masta tri vent which is what I think you are on about?? They are good rugs but again be careful with size.The problem with these wide but not tall types is that although they need the room around the front,every where else get's swamped if the rug is too generous and deep (I now avoid premier equine like the plague for this reason,came down to my lads knees and past his hocks,although fitted well around the front).I had a masta for my clydie mare and was the only rug that fit her in a 6'9",everything else had to be a 7'0",so am guessing they are cut quite generous from this,but maybe they have changed the design by now??
This is another reason why I like the amigo,cut smaller than other rugs generally,but nice design around the chest and shoulder area.
On the lightweight thing,be a bit careful with this depending on when you want to use it?? They are summer turnouts for a reason,as they are designed to keep a horse dry,not warm.They flatten the coat,and stop them being properly 'insulated',so will actually make a horse feel colder than if they went without in low temps.I am using one now as it's still quite warm,but won't use one after end of this month or before if it get's very cold.Would be better and more effective to get one with a small amount of filling (can get ones as low as 40g),or even a mediumweight for really cold weather (my lad will wear a 200g rug from december onwards),They are designed to be breathable,so won't need to worry about over heating,unless it's really warm and sunny of course!!
If he's a good doer (I am guessing he is from his pics
),and you are not clipping,maybe you could leave him without a rug at all??
Sorry for waffle,I have a bit of a rug obsession