Hi H., I think it depends on the tooth and your son, too. My son's teeth always seem to take a long time to come in once I've seen the bump... I'm talking weeks, sometimes more than a month. He usually seems a lot calmer once the tooth finally cuts through the gum.
The white spot on the gums could be a tooth (it's hard to say without seeing what you're seeing)... or it could be thrush. Look and see if he has white stuff on his tongue or cheeks that doesn't come off if you rub it. That usually means thrush.
Wow on his growth! My son is a year and a half and still wears size 4 diapers (size 5 at night with diaper doublers). But my son was the complete opposite of yours. He was born 8 lbs, 1 oz., but his growth rate was pretty minimal for the whole time I was breastfeeding. Once he went to table foods and whole milk he started growing like a weed. I say as long as he's healthy don't worry too much about the growth (especially if you and/or your husband are tall). My friend's son was always in the 90th+ percentile until about 9 months old and then he started slowing down and is now below average. My son at a year and a half is just above average in height and just below average in weight but was often in the 10-25th percentile when he was little. I don't think it really means much unless there's some underlying health issue.
Keep in mind that the fussiness may not just be due to teething, so try not to over medicate him just because he's fussy. He may be overtired or gassy or anything. I like the Hyland's pills but keep in mind their active ingredient belladonna can be dangerous (especially since some people notice the effects at lower doses more than others) and has lots of negative side effects. Tylenol, while mostly safe, can also harm your organs if given frequently enough, so by all means give your son relief if he really needs it, but try to be conservative (which it sounds like you are), especially at three months old.