If the father is alive even then the son is allowed to do the ancestral offering for his deceased grand father, great grand father etc..

I thought that this was not allowed but see the following verses:

Manu Smriti 3.220. But if the (sacrificer’s) father is living, he must offer (the cakes) to three remoter (ancestors); or he may also feed his father at the funeral sacrifice as (one of the) Brahmana (guests).

3.221. But he whose father is dead, while his grandfather lives, shall, after pronouncing his father’s name, mention (that of) his great-grandfather.

If the father is alive even then the son is allowed to do the ancestral offering for his deceased grand father, great grand father etc..

I thought that this was not allowed but see the following verse 3.220:

Manu Smriti 3.220. But if the (sacrificer’s) father is living, he must offer (the cakes) to three remoter (ancestors); or he may also feed his father at the funeral sacrifice as (one of the) Brahmana (guests).

3.221. But he whose father is dead, while his grandfather lives, shall, after pronouncing his father’s name, mention (that of) his great-grandfather.