I have been corrected in this many times by both my vet and an equine nutritionist. You should be able to see 5-6 ribs on a weanling, at least. (For any breed, this is REALLY true of Drafts though as obesity is a major issue with them)
What you do want to make sure of is that they do not lose so much weight their spine is prominent, that the base of the tail loses too much (should still have a "round bum") or that they have NO neck fat. Don't go by ribs alone - they can be misleading, especially in the deep chested and big breeds.
I know people freak out when they see a horse which is actually a good weight, we're just not used to seeing a "trim" horse (and it upsets us) - but being lean is so much better for them while they are growing than being fat. It will reduce the chances of him developing OCD, arthritis, and developing too quickly.
That aside, we do not generally feed our drafts grain at all - if they need more than pasture/forage we give it to them in the form of a good quality hay. I've heard the many many people say not to feed "rich" hay to drafts, but, we found they do require a certain amount of protein (which is a bit higher than "crap" hay) to do really well, especially while growing. Ours seem to do best on an alfalfa/grass hay mix.... and what they DON'T need is all the sugar that usually comes in the low grade grass hay (think of it as feeding a child candy for meals instead of healthy food... the health ramifications are HUGE - not necessarily immediately, but later in life).
If you're finding he puts on too much weight with a good hay, then choose a lower quality hay... but do make sure to watch how much sugar is in it. The later the hay is cut, the higher the sugar content will be.
The odd time we need to supplement the hay and forage diet - we do so with a low carb (low sugar), high fat pellet, and a low-sugar(or sugar free) beet pulp (yup, they now make it.... that's how aware people are becoming of how much sugars they're feeding their horses, and how BAD it is for them!)
Drafts can be prone to obesity, OCD, Arthritis (later in life), ESPM/EPM, Cushings, Founder/Laminitis - the risks of all of these can be reduced with a solid breeding program and monitoring against obesity.
(and again, this comes from our vet and certified Equine Nutritionist - both of whom have taken me to task for allowing our Clyde mare to regularly be too fat!! (she's a SUPER easy keeper, and doesn't do a lot of work yet... we call her an "air fern") )
BTW - congrats, he's adorable!!!