What Emma said!
Also what type of sippy are you using? If it's a valved non-spill one try a free flow one instead, they are much easier to get the hang of. I used the Tommee Tippee basic flip top cup with my kids, I did start with the non-spill for DS until I tried them one day, I couldn't get anything out of it!
Some parents do manage to switch to cups ny 1, but the majority of us keep bottles for a while after. My DS had a bedtime bottle until 2.5 whereas DD switched to a cup at 18 months with no problem. Both were off the morning bottle by 18 months. Don't worry too much and just let him keep practising with the sippy, he will be able to switch at some point!
Laura