Congrats on your new Yorkie!!
Regarding the amount of food he should be eating, here's a link to a food calculator I use all the time...
Dog Food Calculator
You just enter your dog's weight & the calories per cup of food...the rest is magic
Mine never eat the amount suggested on the bag. So this is a great way for me to figure out how much they really need.
There are some good step-in harnesses that don't need to go over the head. One of those might be easier for you to use. We have both a Buddy Belt & a Choke-Free Harness...both go through the legs & buckle or velcro between the shoulder blades.
One of our members (Maximo) sells Buddy Belts with a discount for YT members.
Buddy Belt Harness
LilSis has a discount on the Choke-Free harness.
Ultra Easy ChokeFree & PawRider Pet Seatbelt Adapter (USA)
Maybe he had a bad experience with a harness before you got him? Have you tried using treats to lure him into it? I saw a great YouTube video with Chi where the treats really helped. I'll try to look for it & post a link if I find it.
Good luck!!