Don't worry about him drinking out of a open cup too much. Every baby/toddler is different in how fast he/she picks up a new skill. I introduced an open cup, sippy and straw cup as soon as both of mine took an interest in them (4/5 mos). DD was a fast learner with learning how to drink from an open cup but ds took a lot longer.
The thing with drinking and eating witht he proper cup/utensil requires lots and lots of practice. Very much like learning how to roll/crawl or walk.
You could put bm in the cup/sippy but beware that there may be aware that there may be some wastage. Personally I would let him practice with some water first and then as soon as he figured it out put in bm.
As for not eating his meals, then offer him bm if he is hungry. There's really no point in making him go hungry. Sometimes bm is just what he needed when he's sick or teething.