G.M.
Any reason why he needs 'regular' cow's milk in his diet? You could just cut out the daytime breastmilk bottles and keep breastfeeding him when you are home. At 18 months, he should be OK with just nursing morning and evening. If you are worried about vitamin D, just make sure he's getting enough sunlight exposure (this is just the right time of year for you!). If you are concerned about calcium, just up his intake of yoghurt (plain or mixed with stewed fruit) and cheese.
Also try goat's milk, which is MUCH better for humans than cow's milk is. I know that Kathy V, a regular Mamasource poster, is really well-versed about this, so get in touch with her.
I would respectfully urge you not to give a child that young any flavoured milk. It's full of sugar or, worse, high fructose corn syrup, and any benefits of milk would be lost in the negative impact of that much sugar.