Bootloader 5.3026 is too old for SDHC cards. He needs 5.5022 or newer. Best one to get is the 5.5128 or 5.5136. Worst one to get is 5.5130.
Get the 5.5128 from the internet or from Tomtom itself. I would prefer the latter option since I know it is a trusted source.
Download the above cab, open with Winzip or 7-zip or Windows explorer and extract the file called SYSTEM. (Not TTSYSTEM)
Copy this to the OLD (i.e. SMALL) sd card and reboot the tomtom on it. Make sure that the battery is fully charged before attempting this. The bootloader will be upgraded and after another reboot the tomtom accepts SDHC cards op to 32 GB and higher.