Just to update everyone: We went with the Prince Lionheart booster seat recommended by Ashley (we got this one: https://www.amazon.com/dp/B005ZBHXOA/ref=cm_sw_r_apa_i_ldRSEbT5TBPPG), and it's been great. Very straightforward and easy to use and the perfect height between our chairs and our dining table. What helps is that we already had a step stool for him (Greenco super strong foldable step stool, here https://www.amazon.com/dp/B00O9A57MS/ref=cm_sw_r_apap_hbspyXMtw9fUU), which has let him to work at the counter with us, brush his teeth and wash his hands in the bathroom, and all that, and since he can carry it, he can set it down to make getting in and out of the chair with the booster seat easier.