MS-DOS (FAT32) USB to MS-DOS

I am attempting to set up bootcamp for my 2013 MacBook. I have a 16gb USB for the Windows ISO. The USB was formatted in MS-DOS (FAT32). I need just MS-DOS for bootcamp. I attempted to switch it using Disk Utility but when i erase and select MS-DOS it just erases and reverts back to (FAT32)? any way how to fix this?

For all practical purposes MS-DOS is MS-DOS (FAT32) nowadays, see https://www.howtogeek.com/235596/whats-the-difference-between-fat32-exfat-and-ntfs/ for instance:

FAT32 is the oldest of the three file systems available to Windows. It was introduced all the way back in Windows 95 to replace the older FAT16 file system used in MS-DOS and Windows 3.

[...]

Limitations come with that age, however. Individual files on a FAT32 drive can’t be over 4 GB in size—that’s the maximum. A FAT32 partition must also be less than 8 TB, which admittedly is less of a limitation unless you’re using super-high-capacity drives.

I need just MS-DOS for bootcamp.

No you don't - you want FAT32 - this is what macOS formats your drive when you pick MS-DOS and what you should use. According to Wikipedia the original 8-bit FAT doesn't use a partition table, can't deal with directories or files bigger than 8MB, and was for use on 8" floppy drives.

Apple do say here to use "A blank 16 GB or larger external USB 2 flash drive, formatted as MS-DOS (FAT)" but they are either using FAT in the general rather than specific sense (which is confusing) or the document is wrong.

I attempted to switch it using Disk Utility but when i erase and select MS-DOS it just erases and reverts back to (FAT32)? any way how to fix this?

No there isn't. As your USB is 16GB it will always format FAT32. On macOS a volume less than 5MB formats as FAT12, from 5MB-512MB as FAT16 and bigger than that as FAT32.

These are the (Microsoft) filesystems macOS can identify (from diskutil listFilesystems)

-------------------------------------------------------------------------------
PERSONALITY                     USER VISIBLE NAME
-------------------------------------------------------------------------------
ExFAT                           ExFAT
Free Space                      Free Space
  (or) FREE
MS-DOS                          MS-DOS (FAT)
MS-DOS FAT12                    MS-DOS (FAT12)
MS-DOS FAT16                    MS-DOS (FAT16)
MS-DOS FAT32                    MS-DOS (FAT32)
  (or) FAT32

From Understanding File System (FAT) these are the various size limits:

Differences between FAT12, FAT16, FAT32

• a FAT12 file system contains 1.5 bytes per cluster within the file allocation table.
• a FAT16 file system contains 2 bytes per cluster within the file allocation table.
• a FAT32 file system includes 4 bytes per cluster within the file allocation table.

System  Cluster limit
FAT12   The amount of data clusters is less than 4087 clusters.
FAT16   The amount of data clusters is between 4087 and 65526 clusters, inclusive.
FAT32   The amount of data clusters is between 65526 and 268,435,456 clusters, inclusive. 

The macOS commands that create filesystems such as diskutil and hdiutil do not allow you to specify a type of FAT but rather infer it from the size of the volume.

For example using .dmg as I don't have such small disks:

• hdiutil create -size 0.1m -fs MS-DOS -volname 01M /tmp/01m.dmg creates FAT12
• hdiutil create -size 4m -fs MS-DOS -volname 4M /tmp/4m.dmg creates FAT12
• hdiutil create -size 5m -fs MS-DOS -volname 5M /tmp/5m.dmg creates FAT16
• hdiutil create -size 512m -fs MS-DOS -volname 512M /tmp/512m.dmg creates FAT16
• hdiutil create -size 513m -fs MS-DOS -volname 513M /tmp/513m.dmg creates FAT32