Hi, H.. There could be dozenss of reasons why his nursing may have declined - end of a growth spurt, tooth pain, enjoyment and filling up on solids, simply getting busy and distracted, etc.
When he ends a nursing session, does he simply let go? Or does he struggle and sit up and want off of your lap. If he jus tpulls away, it could be mouth pain or it could be some bad habit he picked up. When my youngest was about 4-5 months, she startted to do this weird thing where she would suck for 2 minutes, and then suck suck, let go, such suck let go. It was exasperating. My LC advised changing the routine in every way I could. If normally I nursed lights ooff with in a cradle hold, then she said to strip her clothes turn on lights and music and feed her upright or football hold. It worked - it got her out of the bad habit and I have nursed her for 12 more months. So maybe try to change the habit of him letting go. Have you considered having a brief consult with an LC? they can be magicians.
At that age, especially wiht a physically advanced child, it is possible he just has better things to do. He may drink during the day for thirst and at night for nurishment. He may be getting more milk in the evening now that his day nursing is declining. Perhaps, make the daytime nursing a more sedate peaceful less on the go event to minimize distraction. Is he getting other liquid - milk/formula or water from a sippy? If so, that's great because he needs to stay hydrated, But don't let him fill up on liquids or solids before nursing.
My older daughter self weaned at 13 months, but she was in day care and was bottle fed a lot. She just stopped asking and it was traumatic for me because it ended abruptly. So enjoy the time you have with him now, and be proud you did this great for so long. My second daughter is almost 18 months and I wish she would wean!!! She will go several days in between asking. I am happy to oblige, but it's hard to know what she's thinking. So, every baby and situation is a bit different.
Odds are he is simply cutting back, not stopping. If you are worried about his intake of milk you could pump or offer formula. Soon, you will introduce milk anyway, and the feedings will be cut back because of that. Look at it this way, isn't it easier on everyone for him to make the decision, then for you to have to wean him? Enjoy him and good luck. :)