I wouldn't say there is any harm in going to a cup if he seems most receptive to that. I agree with the sippys without the valves, that way he gets immediate results. There is also a Nuby sippy, which is kinda like a bottle, has a soft sillicone top. You can get them very cheap at Walmart. I know my son would only take milk from a cup if it was warmed beforehand until he was older.
If you haven't tried other types of nipples, that might be worth a shot too, just cause it will be less messy to deal with at daycare, esp if you are using frozen/expressed BM. Good ones that work with breastfeeding are Playtex natural latch (latex not sillicone), I think Dr Browns makes a nice wide nipple, Avent (but those are very hard and don't give like a breast would) or they have a new one called Second Nature that has mutiple very small holes in it that express milk more like a breast.