my dd has found a normal cup (no teat or spout) the easiest to drink from, she got quite damp to start with but a bib with plastic on the back and fabric on the front really helped. it seemed hopeless at the start but i perservered and now she drinks a decent amount at each meal time.
you could see if any friends have differnt types of cups you could borrow so that you can try things out without having to buy loads, your ds might take to one really easily that you might have picked in the shop, plus you won't have wasted too much money.
bec xxx