J.S.
OK as I understand it canker sores are typically caused by herpes virus. Best way to avoid a little one getting infected is to not share any cups, toothbrushes, food etc w/ anyone, and especially not with someone else who has ever had a canker sore. So if you have ever had one, don't share a cup w. your little one as you have the virus. Once you have the virus you will always have it. People think I am nuts as i do my best NOT to let my 21 month old share cups with anyone but I figure the longer I keep her herpes free, the better. Also, my pediatrician recommends no more than 4 oz of DILUTED juice/day ie 1 oz of juice plus 3oz of water. We only give it on weekends, otherwise she drinks lots of water and milk at mealtimes. If he's already used to juice you might have to dilute it gradually for him.
As for dressing at night, here's my method of determining what to dress her in. I check weather.com for the overnight low temp and the hour by hr to see how the temp looks in her sleep period (8pm-8am). If it's going to be under 60, I use layers. A thin pair of two piece non-footed pjs under and another pair over. The weight of the second pair is determined by the temp. If under 50, definitely the second pair is fleece. If mid to high 50s a light second pair w/feet. If it's hotter (like Thur/Fri night) I put shorty pjs on her and lay out a second longer pair. I check her around midnight before I go to bed and if it's cold in her room put the other pair right over the ones she is wearing to minimally disturb her seep. We also have a ceiling fan which I'll put on if it's warmer and then just turn off before i go to bed to make sure it doesn't get too cold. We always have two fleece blankets in her crib so she can cuddle with them but she really isn't adept at covering herself properly yet.
So, my point is that you understand what your son is like, so I recommend you gather the appropriate data on the temp etc (and factor in how your house cools/heats inside in response to outside changes). Ok maybe it sounds complicated but it isn't, and we usually end up with my little one very comfortable.
Hope that helps!