Sometimes means 'on certain occasions', 'somewhat frequently, but not always'.
- I sometimes go swimming on the weekend.
- Sometimes I forget to take my umbrella.
Sometime means 'at some point in time'. It's used for a single occasion (where 'sometimes' suggests frequency), and is usually used with a second adverbial expression of time.
- You need to finish this report sometime in the next few days.
- She should arrive sometime soon.
- They're going to buy a house sometime in the near future.
I think that depends on how many times you want him to listen to you. If you want him to make a habit of listening then the first one is correct. The second alternative means 'One of these days you should, just for once, listen to me'. I say this with some hesitation, since I'm not sure if the American usage is different.