I feel like if I invest half the amount in Bitcoin for a year after receiving his retirement allowance, I might be able to hand over more money to my dad at the end of the year.
Hope I get proper advice from you.
This should be an agreement between you and your dad; you can't rely on your opinion and that of others here because we are not going to share the risk involved with you. It's risky for you to invest up to half of the funds in Bitcoin; instead, you can discuss with your dad that even if 1–10% of it can be invested over a long period of time, it reduces the amount at stake, and you can as well keep a close watch on the price movement since the invested amount will be less. There's no need to panic sell; you can wait patiently for the price to go up in order to realize a profit.
I would suggest to make diversification so less risk to lose all
What do you mean by "diversification"? Does sharing the assets with different altcoins change the fact that crypto investment is risky for such a kind of money, which seems to be the family's live savings?
Your dad serves in the army for 25 years, it seems you need to live through the horror of war before you understand how hard that must have been for your father, now you want to risk all that he ever worked for?
The OP doesn't need to undergo anything in order to understand what the pension means to the father; a reasonable and close child to the parent will understand all that it took the dad to make a living; they share the pains indirectly, and the OP is not just taking all the father's retirement funds to invest in Bitcoin; all he/she asked for was an investment idea in order to increase the father's retirement fund so that it can cover up the expected expenses. Even if not all, at least a little thing will be added to it; it's better than nothing.