My son has a similar issue around the same time. He is 10 months now and goes every other day. But for about 2-3 months, it seemed like he'd only go every three weeks. I read that as long as his bm is not hard when it comes out, and as long as he is not struggling to go and/or in pain, then it is not constipation. Eventually, despite how difficult this was for me to watch, he became regular again.
I noticed that there were certain things i ate that seemed to set off the whole problem. I drank some kefir yogurt, which eventhough is very good for both of us AND seemed to increase my milk supply like crazy, thats when his movements changed.
I spoke with our doctor and they said he was fine though.
One thing that i think helped him become regular again was the introduction to Natren Life Start, which is a probiotic (b.infantis) for infants. This is actually recommended for infants in the book "Digestive Wellness for Children."
I just put a 1/4 teaspoon in a little water and fed it to him with a medicine dropper. I did this for about a week and it worked immediately. A breastfeeding mom can also take some herself. Its a bit pricey(ranges from $16 - $21) but it is well worth it. Keep it refrigerated.
I highly recommend that book mentioned earlier, as it talks about childrens nutrition and illnesses from a standpoint that you most likely wont hear from anywhere else.
I hope you find some resolution.