Good thing you said your feet "are not too ticklish" instead of "are not ticklish at all". 🙂 You are already limited to how ticklish they are, but there are a few things you can do to improve the situation.
Prepare your mind and think of your feet to be very ticklish. The more you psyche yourself up, the more ticklish they will be. (The opposite works too, btw. ;-) This is a poor man's self hypnosis. I would not go for anything more than this. As others pointed out, I am not a big fan of other people messing up with my mind. Keep it real!
Try being blindfolded or in a position where you cannot see when/how/where they are tickled. This goes together with the previous recommendation. Anticipation may also improve sensitivity, because your mind will be doing extra guessing work.
Make sure your feet are warm before they are tickled, e.g., hair blower, foot massage, or right after you take your shoes/socks off (as it was previously suggested). Cold feet are usually not as sensitive.
Keep them soft and sensitive. Someone already provided a link to this forum for that topic.
Some people are more sensitive when their feet are wet. Try that.
Try different materials around them, e.g., naked, thin/thick socks, nylons. And also try different tickling instruments, e.g., feathers, fingers, nails, brushes.
Whatever you do, make sure you and your boyfriend both participate in this research. Make it part of your tickling fun every day or however often you decide to do it. For instance, today try lotion, tomorrow try brushes, the next day try nylons, etc. I am sure this will keep the both of you entertained and you will learn stuff about yourselves in the process.