Yes, a Jain can eat pumpkin.

In Jain dharma:

• Pumpkin (kaddu) grows above the ground, so it is not a kand-mool (root vegetable).
• Jain food rules mainly forbid root/underground vegetables (potato, onion, garlic, carrot, radish, etc.), as well as meat, eggs, alcohol, honey, and fermented/extra-harmful items.
• Pumpkin is therefore generally allowed for both Śvetāmbara and Digambar Jains as a normal sabzi, just like bottle gourd, ridge gourd, etc. ( jainknowledge.com)

Even during special periods like Paryushan or Chaturmas, simple non-root vegetables such as pumpkin are usually permitted, unless your guru, family, or local sangh follows a stricter rule. ( jainknowledge.com)

For a simple list of commonly allowed vegetables (including pumpkin), you can read more here