One thing to recognize is that nerves in one location which provide you a welcomed sensation may not provide the same response in your partner. Stimulation of the anal area for you may be very arousing but for your partner it may be a total turnoff. Take baby steps rather than just jump into the dream. While it is tempting to go for the gold so to speak, break the plan down into baby steps.
First you likely already know a repertoire of what he finds pleasurable. Of course he enjoys when you handle the penis, scrotum, stroke the inside of his thighs etc. Try touching the perineum (the skin surface between the scrotum and anus). Many males enjoy stimulation of the area bot the soft touch of a finger moving along the ridge and actual compression of the zone. Develop a step by step plan of sequential areas where you work around trying various touches and pressing as you circle the final target, incrementally working towards the final goal.
Assuming you find a positive response to stroking or massaging the perineum have him roll over on his stomach. Sit straddle of his buttocks and begin massaging his back using body oil or lotion. Initially confine your hands to the zone between the waistline and neck/shoulders. After five or ten minutes have him roll onto his back, and again sit straddle his legs but leave the space below the waist accessible. Continue to apply lotion or oil from his neck line down past the waist and around the genitalia. Work your way down the top of the thighs and slowly begin massaging the inside of the thighs between the knees and genitalia. Work your way around the outside of the thighs also. While doing this make mental notes of his responses to touch in each area.
Move back to the inside of the thighs. If he is enjoying the massage up to this point push your fingers between the thighs and stroke the skin just after emerging to the back side of the thighs moving up and down between the knees and genitalia. Observe for changes in his response to physical contact. If he is enjoying the massage thus far have him roll back over on his stomach. This time begin massaging at the waist and down the buttocks. Avoid moving towards the anus at this time. Observe for changes in his response as you begin stroking the buttocks. Now as you did earlier while laying on his back move your fingers down between the thighs but continue to stay away from the anus. If he is still enjoying the stimulus continue stroking/massaging the buttocks and thighs. Stroke the skin where the buttocks begin to roll into the anus. Carefully observe the response. If you discern he is enjoying your scouting thus far continue to move further into the skin near the anus. If you are having difficulty discerning his response lean forward and ask him if he is ok with what you are doing.
If he indicates that he is enjoying the massage thus far add lotion to your hands and continue to move deeper into the crevice between the buttocks. When you decide to touch the ring of the anus just brush it, do not dwell. Gauge your partner's response before continuing. If you determine he is enjoying what you are doing run your index finger around the anal ring. If you have been using lotion which is oil based you may want to wipe your hands with a towel and switch over to a water based lubricant such as KY Jelly or other brand. Put on a glove or finger cot. Apply KY and continue running your finger around the anus. If he is enjoying your stoking the area he will likely be making sounds to alert you. If not ask. Once you have made it to the anus with a positive reaction you can move onto enema administration.
Just like the the process of determining your partner's response to anal stimulation break the enema down into baby steps. Initially gauge his reaction to inserting the probe end of a digital thermometer or similar. When you move to injecting water use maybe eight ounces. Once he has evacuated the contents try a shower together where you can take him to a climax. When you detect he is about to ejaculate insert your index finger slowly, locate the prostate and vigorously massage the gland. The intensity will surprise both of you.
Whatever you are doing whenever, if he says stop, then STOP!