You need guidance from a local tax and/or legal professional for this matter. The issues can become quite complex.
The expenses you paid on your son's behalf (for his business) would be considered gifts unless you and he had a partnership agreement, in which case your payments would be considered contributions to capital.
You can organize and register an LLC with the state, but the default classification would still be a partnership for federal income tax purposes. The agreement may specify that you and your son share in the profits and losses on a 50/50 basis, but you will not be able to deduct your share of the net losses (from your ordinary income or capital gains) unless you materially participate in the business (and it does not appear that you do so).
See https://www.irs.gov/publications/p925#en_US_2021_publink1000104581
An LLC can be organized regardless of whether or not the business is profitable and an LLC can be organized regardless of the month in the particular tax year. However, note that the default classification for multi-member LLCs is a partnership for federal income tax purposes and, as such, the LLC must file Form 1065 and issue K-1s to its members (which, again, will most likely require professional guidance and/or tax return preparation).
See https://www.irs.gov/instructions/i1065#en_US_2021_publink11392vd0e555
Again, seek guidance from a local tax and/or legal professional for this matter.
