Unfortunately your post comes over more as being negative and judgemental rather than concerned. However you are clearly concerned as well, so putting my own judgemental comment aside ..... you have only had two visits so it might be wise not to be hasty. What constitutes a well cared for child? As he is 19 months I would expect him to be walking, saying some words, drinking from a cup rather than a bottle, able to feed himself. Is he growing well, got healthy looking skin, making eye contact, stacking toys, waving, clapping and does he laugh and giggle when you play peek a boo, chase him etc? Is he clean and appropriately dressed? There is nothing wrong with him eating several small meals rather than three large ones; what is more important is the nutritional content which you cannot be sure of. Are his teeth well looked after?
Certainly I would be concerned about the sweet smell on his breath and the simplest answer is for his dad to take him to his gp, baby clinic or pharmacist. If you put aside your feelings about the mother and concentrate on making the best of the contact visits, good healthy food, playing, singing and starting to look at picture books then you will be doing what is best for your grandson. As far as naps go, by his age he may need longer naps some days than others and I wouldn’t worry about that. Make the best of his visits and enjoy his company. If you can build a reasonable relationship with his mum, putting aside your reservations, you and his dad will see more of him and be able to be a positive influence on his life. Good luck.