You could encourage him to accompany the clitoral stimulation with some fingering in the vagina. It is possible to perform cunniliguis and insert a finger or dildo into the vagina at the same time. It may not sound possible but it is Also, alternating clitoral and vaginal stimulation helps. Using the thumb and forefinger - one on the clitoris and the other in the vagina (massaging the g spot if he can find it) is also a good technique.

For occasional fun, the use of vibrators can be good. Remember that you can use it on him as well as him using it on you

It may be that you could benefit from tantra techniques as well. A lot of them are about delaying and prolonging orgasm. Things like genital massage can be a lot of fun