You should be nervous. Constructive criticism is still criticism, and this particular topic is supremely sensitive and tied heavily to ego. The first thing you need to remember is to be mindful of delivery. How you communicate this to your partner is just as important as what you communicate to him. Another factor is timing; what you decide to say must be tailored to what you are doing at the time. The type of conversation you have with your partner is determined by whether you two are in the heat of the moment or if this is a conversation occurring outside of sex.
The next key component, as clichéd as it sounds, will be to focus on using "I" statements; owning your desires and asking for what you want does not come across as criticism as much as focusing the conversation on what he isn’t doing.
Next, ask yourself what it is you are looking for. What other areas do you wish he’d explore, or spend more time on? Do you know what brings you to orgasm so that you can clearly convey this to him in some fashion? If not, consider investing in some play time alone. If there have been previous experiences (both with a partner or without) that were particularly awesome, think about why they pleased you so much.
If you are in the heat of the moment, there are several ways in which you can tell your guy you want an orgasm without actually talking about it. Positively reinforcing something he does well while he is doing it is one rather sexy way to do this. Say things to confirm you like what your partner's doing, ask him to keep going, perhaps even tell him you are really close an, "OH BABY, please don’t stop now." That way, he gets to feel like a stud (without ever knowing he wasn’t).
Playing a fun game of self pleasure in front of your partner and telling him that he cannot - under any circumstances - touch you while you are touching yourself is another way to show him how you like to be handled. If you can, tell him what you are doing while you are doing it so he hears and sees how much you like certain areas to be touched. Find out how long he can hold out before he can’t stand being a bystander anymore. Under pressure, there is always the option of doing it yourself; nothing wrong with giving your partner a hand (pun intended).
If you decide to have this conversation outside of sexual activity, please consider your words very carefully. As I stated earlier, this is a sensitive subject tied heavily to ego. There is always a chance you'll hurt his feelings. If the conversation goes badly, he is going to have more than hurt feelings. You could try opening the conversation with asking him what his thoughts are on your sex life. Does he feel his needs are being met, or are there things he may want to try but it hasn’t come up or he’s a little too shy to ask? By opening yourself up to the criticism first, and giving him a chance for expression, you can take some of the pressure off when it comes time for you to express your needs and wants. If he says everything is fine, or you don’t want to ask him first, perhaps have the conversation as though you are just making suggestions of things you thought sounded fun/interesting/etc. Do NOT say anything to the effect of previous sexual encounters with other men; you are asking for trouble.
Whatever your approach, keep in mind that this is someone you care about. How would you want him to bring up a conversation like this if the situation was reversed? That will help you approach this in a way that's caring rather than critical.
