I'm afraid that would be VERY difficult. That plant would normally reach about 4-5' long and begin flowering in about 6 weeks. You'll be hard pressed to give it enough light, and it would require quite a large pot. Better to stand by with more seeds and help him plant in May.

George

If you are desperate you might be able to slow its growth down by keeping the temp low. But, how long before you are getting consistently warm temps in MN? I would think you have to make it all the way to May, correct?

If you have a greenhouse, you are in business, although it would end up taking up a huge amount of space. You could trellis it indoors to save some floor space.

I would imagine a cold frame would be both way too small and way too exposed, even at this point in the year to do much good.

Looks like the best option is to teach your son a lesson about planting time. I understand. I have had the itch to garden since December. I've occupied my time with plans, varieties, and architectural additions to my garden (new boxes and trellises this year!). Every opportunity is a chance to learn something. Don't let this one get by without your son learning some valuable lessons. He apparently (with guidance) has successfully gotten a plant to grow from seed. Depending on his age, that can be HUGE! If he's older, you could break apart the roots later and check the growth pattern of the roots. Everybody sees the tops, but the roots are usually out of view. You could teach him about the role of the roots and how truly vital they are to plant development and fruit set.

While he may be upset with losing his pumpkin, you may be able to really ignite a passion for gardening by showing him elements of growth that he wouldn't normally get to see.