The location of that bump looks to the normal part of the anatomy called the caudal peduncle and nothing to worry about....
If he is having swim issues it could be related to several different things-Some it is related to how they are reared at the fish farm and it is a deformity or poor husbandry at the fish farm. It could also be due to poor care at the fish store....
Since he is new-what I would recommend-keep the water quality high with the water changes-your schedule looks good, however, I would add an extra 50% water only for the first week. Provide good nutrition-since he is new-its not uncommon for them to not eat well for the first week. If you have access to mosquito larva-offer some rinse to see if you can stimulate his feeding response. If you have a tannin source-add this to both the tank and the replacement water used for water changes.
Tannin source-IAL (Indian almond leaf) usually you have to get this online
Oak leaf-this is what I use....collect naturally dried and fallen from the tree Oak leaf...add 2-3 to the tank and 2-3 to the replacement water a couple of hours before you make the water change to lightly tan the water as a general tonic.....
If he doesn't improve in a day or so or he get worse/change in symptom-he may need Epsom salt/tannin treatment....
Lovely fella....keep us posted....