My DH is putting a five foot wall (like a privacy fence, but made of 2x6's) on the stall so that will reduce the wind and serve as a kickboard later.
You are putting on the 2x6's HORIZONTALLY, yes? Not vertically? They absolutely must be horizontally, or they will not be even remotely strong enough. Nail or screw them to the INSIDES of the posts, not the outsides; and screw an additional one or two lengths of 2x6 vertically along the outside of each wall, as cleats, at the middle or 1/3 points, so that they are connected together and share the strain.
Also if you are going to use a corral gate as the 4th side of the pen, PLEASE hang it so there is at least 12" of clearance between the bottom of it and the ground, AND very securely attach a sheet and a half of 5/8" plywood so that the gate is now solid. (Use pipe brackets and bolts, or strong wire; make sure the nuts or the twists of wire ends are very very safely rounded/covered/outofthehorse'sway.) I know some people use gates 'plain' as stall doors but this is just asking for a cast horse, and beleive me, at your level of experience and with a new horse, you DO NOT want him getting cast. The plywood will keep him from threading his legs thru and getting 'em stuck; the 12" gap below the gate will keep him from getting legs trapped and skinned down there. The plywood will make the gate extra heavy, so make sure you have a cinderblock or suchlike to rest the far end on when it is in its closed/latched position.
It's still all a really bad idea though. I know the idea is that you'll transfer to a boarding barn if problems start to develop, but I just have to say one last time: problems are likely to sneak up without your recognizing it until they are very hard to fix. Not just health problems but behavioral problems too.
This is not theory, this is having worked with horses and horseowners for most of my 43 years and having seen an AWFUL lot of situations like yours go vigorously and irretrievably south, despite the owners very best intentions
Remember that not all physical and behavioral damage to a horse is realistically fixable, and things can happen so fast you can't do anything about it, or so gradually you don't realize it til it's too late...
Good luck, she said, wincing,
Pat
You are putting on the 2x6's HORIZONTALLY, yes? Not vertically? They absolutely must be horizontally, or they will not be even remotely strong enough. Nail or screw them to the INSIDES of the posts, not the outsides; and screw an additional one or two lengths of 2x6 vertically along the outside of each wall, as cleats, at the middle or 1/3 points, so that they are connected together and share the strain.
Also if you are going to use a corral gate as the 4th side of the pen, PLEASE hang it so there is at least 12" of clearance between the bottom of it and the ground, AND very securely attach a sheet and a half of 5/8" plywood so that the gate is now solid. (Use pipe brackets and bolts, or strong wire; make sure the nuts or the twists of wire ends are very very safely rounded/covered/outofthehorse'sway.) I know some people use gates 'plain' as stall doors but this is just asking for a cast horse, and beleive me, at your level of experience and with a new horse, you DO NOT want him getting cast. The plywood will keep him from threading his legs thru and getting 'em stuck; the 12" gap below the gate will keep him from getting legs trapped and skinned down there. The plywood will make the gate extra heavy, so make sure you have a cinderblock or suchlike to rest the far end on when it is in its closed/latched position.
It's still all a really bad idea though. I know the idea is that you'll transfer to a boarding barn if problems start to develop, but I just have to say one last time: problems are likely to sneak up without your recognizing it until they are very hard to fix. Not just health problems but behavioral problems too.
This is not theory, this is having worked with horses and horseowners for most of my 43 years and having seen an AWFUL lot of situations like yours go vigorously and irretrievably south, despite the owners very best intentions

Good luck, she said, wincing,
Pat