Not really anything to worry about and no set growth expected really as I have the same with my dd,I got more than 6-7months out of shoes for my dd aged 3 now as she is such a slow and small at growing. Be thankful you will save money by not having to buy shoes every 12weeks or less.
You could get your son looked over by hv/gp if your worried but I'm sure there will come a point of growth spurt for your and my dc And then we will pay.
I sometimes worry too as my brother literally stopped growing at a normal rate anyway aged two but then caught up about aged 17-18 so wonder of it can be genetic.
I wouldn't worry, even now at 12 and 8 my oldest 2 tend to stay the same size for ages and then jump a couple of sizes in a very short space of time. My youngest looks like she will be going the same way. It's more of a PITA than a worry now!
Ds is 12 and only goes up one shoe size a year....he still has comparatively small feet - size 2.5 (adult size) for his age....He hasn't really hit a growth spurt but I'm fully convinced he's never going to go above a size 7 - 7.5....
Be thankful...children's shoes are very expensive and to have to buy several pairs of shoes a year, particularly if shopping in Clarks or Start-Rite can become very costly indeed.
Poocatcher dd was similar. She started walking at 10mths and we went to get her measured for shoes, she wasn't even a size 2! Finally at 14 months she creeped into a 2 and could get shoes. I think she made it to a 3 by about 2 years old.