Unasked question: Do you have to file at all? His final business and personal tax returns are to be filed by his personal representative or executor. Is that you? Maybe there is someone else involved with the trust, an executor or trustee? That's the person who is required to file. In some cases, the responsible person for the business return might be different than the responsible person for a personal return. We don't know the whole story, if there was a trust, there was probably an attorney to set it up.
Then, he must file for 2020 and 2021 if he is required to file (more income than certain limits). Basically, if his business has a net profit of more than $400, and if he personally has more than $12,550 of taxable income, although this can get quite complicated.
The type of business return depends on the type of business. If it is a partnership, an LLC with more than one owner, or an S-corporation or C-corporation, the business files a separate tax return and requires the Turbotax Business version. If it is a sole proprietorship or single-member LLC, he would file a schedule C included as part of his personal tax return, this would use the Turbotax Home & Business version.
Even if he had no business income, a return might need to be filed to close the business and dispose of business assets or inventory. Depending on the type of business, he might have assets that were sold or dispersed to his heirs (tools, computers, vehicles, etc.), or inventory that was included in a previous year's costs and either sold at a loss, or given away, or destroyed, all of which might have tax consequences to the business.
If he required a separate business return, the late penalties are substantial. There are ways to request a waiver for cause.
You can request a transcript of his past returns, this may help you gather the information you need. You will need to do this in writing and attach proof that you are his legal representative (if you are in fact, he legal representative for tax purposes).
I agree with seeing an accountant. They can tell you what to look for in his papers and his bank accounts to put together a tax return. You need business records, bank records, 1099s, and so on. If his clients issued 1099s, you can get copies from the IRS.
If audited, you must prove what you filed. If you can't prove what you filed and the IRS says different, the IRS can assess additional tax. This can be complicated. For example, if he had a weekly cash deposit to his bank account of $100 per week which was a gift from a friend for medical help, but you can't prove it was a gift, the IRS could call it business income. Or if he got a 1099 from a GoFundMe for medical expenses, but you can't prove it, the IRS could call it business income. Of course, most people aren't audited.
But here, it really does pay to hire someone to help you. His final returns can be audited for 6 years from when they were filed, and you don't want to get a bill 6 years from now that you would have to pay (assuming his excess funds after taxes were dispersed to his heirs, the heirs would have to pay back any tax assessments.)
