60 miles is a frightfully long first trip for anyone, let alone a 18 month old. My son would have loved it, but one of my daughters would have been scared for life. I'd try a much shorter trip first.
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I've had my daughter on plenty of rides, both sled and atv from that age and younger (first ride was in a snuggly at 6 months). He will fall asleep so be ready. The helmet and neck roll is ok if he can handle the weight. Another option if you need it is a hockey helmet. They come with a face shield and are pretty light weight. Mine has a MX HJC and we struggle with goggles sometimes.
I use a sledpal harness and it works great. My daughter complains about the leg straps, so we don't use them too often, but I wish we did. The leg straps keeps the harness from choking her if I have to stand up for some reason. Luckily we haven't had that situation.
Pack a blanket or something to wrap him in when stopped or if he gets too cold. Also keep a close eye on exposed skin as mine tended not to complain about cold noses etc. I have to watch she doesn't get frostbite. The good news is that in front of you, they are protected from the wind pretty well.
I'm going to try mine in back of me on a 2 up seat for the first time this weekend. She is 5 1/2 and about 44" tall, so we'll see how she does. She may want back in front though as she likes working the gas. When she is strong enough, around 8 or 9, I'll put a throttle block on our old phazer and see how she does.