I cannot confirm anything without examination but it can be friction blisters, heat rash or fibromas.
Place protective covering or bandages over the sore to decrease friction on the skin until the sore heals. Rub sandpaper disks or pumice stone over hard thickened regions. Soak feet or hands in warm soapy water to soften calluses. Then rub with a pumice stone and apply some moisturizer. Make your son wear shoes that fit properly and keep feet as dry as possible. Wearing wet shoes, boots and socks will aggravate the problem. So change socks regularly and use foot powder to help keep your feet dry.
If still the lumps persist, then pls go in for removal by surgical means or with keratolytic agents (medicines that break up hardened areas of skin). Shaving or cutting off the hardened area on the skin by a doctor is also helpful.
I hope it helps. Take care and please do keep me posted in case you have any additional doubts. Kind regards.