Is it safe for a baby to sleep in a portacot long term, instead of a standard cot?

Portable/folding cots are not recommended for permanent sleeping.
The mandatory standards, developed by Product Safety, has the following information.
Folding cots are designed for temporary use.
It is important to use portable cots as per instructions, as they are not as sturdy as a household cots & more likely to develop “wear & tear” concerns.
https://www.productsafety.gov.au/products/babies-kids/kids-furniture/folding-cots
https://www.productsafety.gov.au/standards/folding-cots

Product Safety information predominantly looks at 2 specific areas

wear & tear on the cot when used regularly, including weight of child
concerns about mattress use - do not add extra padding/use only mattress from manufacturer.

wear & tear on the cot when used regularly, including weight of child
concerns about mattress use - do not add extra padding/use only mattress from manufacturer.

Red Nose has the following information.

Only use the firm, thin, well-fitting mattress that is supplied with the portable cot. Never add a second mattress or additional padding under or over the mattress, which has been specifically designed for the portacot, as baby may become trapped face down in gaps between the mattress and the sides.
https://rednose.org.au/article/portable-cots

How old is your son?
Are you still wrapping/swaddling him?
How long has this been happening?
Let me know so I can answer your question more specifically.

Anything soft & padded in the cot can increase risk of baby’s head & face being covered.