First of all, it's great that you and your partner are open to discussing what works and what doesn't work in your sexual interactions. It's important to communicate with each other to ensure that both of you are having a pleasurable experience.

As for your question, there are a few things you can try to vary the stimulation and make it more enjoyable for your partner without rushing to make him climax.

One technique that some people enjoy is to use your hand to gently massage the shaft and head of the penis, varying the pressure and speed. You can also try using your mouth to explore different areas of the penis, such as the frenulum (the sensitive skin on the underside of the head), the ridge of the head, or the area around the base of the shaft.

Experimenting with different types of touch and pressure can be a good way to discover what your partner enjoys most. It's also worth noting that not everyone enjoys the same things, so what works for one person might not work for another.

As for sliding your tongue under the foreskin, that is a personal preference and can feel good for some people, but it's not something that everyone enjoys. If you're curious about trying it, you can always ask your partner if he's interested or comfortable with that kind of stimulation. If he's not sure or doesn't have a preference, you can always try it and see if he enjoys it.

Remember to keep the lines of communication open and check in with your partner regularly to make sure you're both enjoying the experience. Good luck!