The point is not to make bad financial decisions, if only obsessed with collecting a lot of bitcin but we have a little money do not put it into Bitcoin all because it will interfere with long -term investment/collection of bitcoin, until you sell your bitcoin because you don't have money to buy needs You are everyday.
We have to ascertain whether he has any other income apart from receiving $100 monthly from his father, he lives separately from his parents and if he doesn't have any other income then $100 monthly is not enough for any investment, the impact of inflation means we have to save money to meet our daily needs, but if he could allocate $20-30 that would be better invested in bitcoin.
I like OP's motivation for wanting to invest in bitcoin, but I hope he has other income to invest more bitcoin before the next bull run, I suggest he learn about earning income from crypto events and signature campaigns to increase income buying more bitcoin for a long term portfolio.