First of all, you deserve to be congratulated for having breastfed your son for so long. It must have been a lot of effort. However, at one years 7 months, his weight at 10 kg is a trifle low. There is no evidence that neem paste causes fits. You can try to simply communicate to him that he cannot breastfeed during the day any more. Be firm, easier said than done. But it works. He will cry for a day or two but will learn. Children learn quickly. You can even apply neem paste if you want to.
Now you can be giving him fruits, energy dense foods - halwa, kheer, even ice cream (in moderation). Give him wholesome home cooked fresh hot meals - there is nothing in the world better than this - dal, chawal, subzi, roti, etc. While he would be apprehensive at first, be persistent, a little more patience for a few months and your baby will be eating normal home food - minus excess spices!