I prepare by packing my insulin, my BG meter and my Glucotabs.
When I see food I guestimate how many carbs (not "crabs" ... I hope never to see crabs in my veggie meals
) there are in what I eat and inject at the time.
Because I am estimating, I usually just count the big hitters - bread, cake, etc. Due to inaccuracies in my estimates, the rest balance out later. Especially if I am having a dance.
When at home, I usually inject 15 minutes before my meal but doing it on sight once a month is not going to cause diabetes complications.
If it is a buffet, I may decide how many carbs I am going to eat, inject and then select my plate for this many carbs but, as your son is just starting out, I would suggest he just injects for what is on his plate as it is easier.
Then a couple of hours later, I take a BG reading and make a correction if necessary.
Some people use apps such as Carbs and Cals to help with their estimations. I couldn't get on with this method: I am too impatient.
Bear in mind, all carb counting is an estimate. If you are eating a slice of cake when the total carbs is on the packet, this is an average - you may have a slice with more or less saltanas than average; if you are eating an apple which you have weighed and checked the % carbs, this is an average because the climate, soil, type of apple all make a different; ...
And you always have the chance to correct.
My approach to diabetes management is similar to my dental care.
I clean my teeth twice every day: when I get up and before I go to bed. OK, there are a couple of times a year, after a long day when all I want to do when I get home is go to bed and sleep. On these days (which are rare), I may go to bed without cleaning my teeth. This is not ideal as I realise the next morning with the taste of old socks in my mouth but my teeth are not going to fall out because I forgot to clean my teeth once in a blue moon.
Likewise, I am not going to get diabetes complications because I miscounted the number of carbs in a wedding buffet.