Your baby is very normal. First, eating every 90 minutes during the night is very natural, and very common for those who have access to fill their needs. Babies would not do this so commonly if it meant they weren't getting the sleep they need. This can last for another year or a little more. It'll be OK a little later on if you want to try to reduce the night feeds but it's a little early for that at 5.5 months. Don't be swayed by all the comments about sleeping through the night as if it's some golden badge to wear proudly. There is no connection between "sleeping through the night" and better infant health or development. Of course, it's been shown that sleeping too deeply throughout the night is associated with a greater risk of SIDS. Those who feed frequently during the night have far lower risks.
I understand that it's the napping that is your larger challenge. "Naturally," he'd be napping snuggled up and happy in the arms of a grandparent or aunt. Most of us just don't have these natural village accessories available daily. The other natural means for him to nap would be wrapped up on your back, or in a sling. Again, that doesn't always work well for all of the things we need to try to do today. Do learn how to wrap him onto your back, or try a backpack. Moms find it much easier to do housework or computer work with their child on their back than in front, and baby-wearing might be your best answer.
When natural means are unavailable or impractical, we often need to resort to less natural, more creative options. Yup, many find that car rides will do the trick. The motion and confinement mimic being attached to a walking body, but once the vestibular input stops, a child, like other mammals, might instinctually worry that the village is roaming away without him. The electric swing and vibrating chair, as mentioned, work well for some. A travel bassinet, following you around the home, might work very well. Most babies are much happier sleeping amidst the sounds of their family's activities, and studies support the value of this more-natural means of napping. My guy would fall asleep in his backpack on my back but I couldn't always keep it on until his nap was over. If I took him out, he'd wake up. I'd lay the backpack on the bed with him still in it. The confinement made his sleeping mind feel that he was still being held, and I'd leave the door open so he could still hear my activity. Mostly, keeping your napping baby in the midst of activities should make him comfortable enough to know that he's not being abandoned, and thus more comfortable to nap.