It depends on the facility. In federal, the amount is $320 per month ($370 in November and December) on commissary items.  This does not include over-the-counter medications, copy cards, or postage stamps. In state prison, the weekly amounts range from $40-60. In the county jails it is about $75 per week. The private prisons like CoreCivic or GEO also allow $75 per week. 

Commissary is once a week. It typically takes one week before the newer inmates can get commissary. But it is more related to the "day of the week they are designated", for instance if they come into the institution on a Wednesday and the commissary day for him is on Tuesday, then he'll be able to go the next Tuesday. Also, money in their account must be there in advance.

Unfortunately you cannot. Inmates have privacy rights and unless the inmate tells you, you won't know for sure. We can tell you 100% that there is not a whole lot he can buy that would be of special interest to you. The commissary list has many things you'd find in your local grocery or drug store at slightly higher prices. I will tell you that $100 is a good amount and he's probably buying some tuna, some snacks, maybe some shorts

Figure prices to be about 15-20% higher than what it might cost at Walmart or Target. But, it's the only shop in town so the inmates have to shop there. ANy amount of money is like the greatest gift you could send a frined in prison. They will decide what to choose from the long list of food and hygiene items. If you are able, $50 is fantastic. If you can do more, you will really make their month... but

You will need to send money to the inmate's trust account where they will have to purchase the prison-approved radio at the comnmissary. Radios caost about $50. We would recommend sending enough to buy headphones too (about $20) :)

Everyone is different. When I first got there, I was buying all kinds of snacks and candy. I thought having those sweet things to eat, would make me feel better. But the more time I did, the more I got into working out and getting into great shape. That is when my preferences changed from sweet to "protein" which meant packets of tuna, mackerel, salmon and chicken. They run anywhere from $1.25 for tuna to $3.75 for chicken.

You should call the facility first before you do anything and see if they would allow it. In state prison, some states allow you to send shoes or work boots (or order them for the inmate and they are shipped in). In federal you definitely cannot. All extra footware is bought through the commissary. The pricing is very fair for sneakers and boots. The inmate would need money on their books to be able to acquire them. 

Commissary is usually once a week. Each prison system has their own rules for choice of available items and the weekly/monthly spending limits 

Most facilities where there is colder climate give the warm winter clothing to the inmates, they do not have to buy these items. If that is not the case, the price should be between $20-$50.

He needs to go and make an appointment with his counselor and let them know what the facts are in the most respectful way. If there is something wrond, they will fix it.