Check he is not having his things taken. Assuming this is not the case, then things you could try might include:
Teach him to count items and to look back as he leaves his seat or where he is playing in case he has left something behind.
Whether or not you think he might have Dyspraxia, look on the Dyspraxia Foundation website for tips on developing organisational skills.
Encourage him to carry duplicates of small item, e.g. pens, and to build in extra time to find things so the knock-on effects are minimised, e.g. to pack his school bag the night before so he doesn’t make himself late and get detention by searching or going back for things in the morning. If he has a phone, get him to set it with reminders that ping at a certain time or when he reaches a certain location.
Teach him how to budget so he understands the value of his possessions. Take him shopping for his expensive items and pay in cash, so e.g. he sees how much money is being handed over for his school uniform.
Rather than getting cross with him, make him do an increasing amount of what is required to retrieve or replace missing items, e.g. searching for it, paying a contribution towards the replacement, being the one who asks for the missing item at Lost Property, make him go with you to buy replacements.
Decide on your "bail-out" policy for secondary - when/ how many times will you solve the problem for him when he loses or forgets something, and when will he have to face the consequences of not having whatever he needs?
Think about safety/ security - does he need emergency cash so he can get home if he loses his bus pass, do you need a key safe rather than entrust him with carrying a house key? Do any particularly valuable items need a GPS tracker tag and/ or insurance cover?