I agree it is most likely he'll be found in the river, his phone died at 3.51am.
The question is how and why he would have gone into the river.
Accounts indicate he was not drunk when he and his friends left the bar so it seems unlikely he would have been acting the fool, eg trying to walk along the guardrail, and fallen in.
He might have gone for a pee in the river but I can't see that he'd have done that from the bridge itself. He was too sober for that. He might have gone down the bank to the river's edge for a pee and slipped in, but because he was pretty sober I would expect him to have struggled and shouted for help for at least several minutes unless he struck his head when he fell. There may have been nobody about to hear him but it would probably have been quiet at that time of the morning so his voice might have carried for a while.
The issue with this is that there would have been no need for him to go near the water to pee without being noticed at that time of the morning because his route after he left the bridge shows roadside trees and shrubs that would have provided cover. This is a view from the road beside the bridge looking towards St John's Campus:
Google Maps
As posted earlier, he might have dropped his phone after sending the last message and if it landed on the river bank he might have gone down to retrieve it and slipped in. Same considerations as above.
Could he have been attacked and thrown or pushed into the river? It's certainly possible, and we are seeing far more violent attacks on random strangers than I can remember before, but I think that would have created much more of a disturbance than him simply falling in the river, with a lot more shouting and so on.
And that leaves suicide. From reports he was now a week into his course. The previous week's Freshers' Week fun was over and done with and actual study had now started. Was he simply already overwhelmed by the step up from A Level to university level study and felt he'd made a mistake but was worried about "letting down" his parents if he dropped out?
Of the above, I'm inclined to discount attack since it should have created a fair disturbance but nobody heard or saw anything. I think falling in the river by fooling about or going for a pee, which would be top of the list if he had been really drunk, are less likely because he was pretty sober. So a retrieving-his-phone accident or suicide seem the most likely to me. However unless his body when they find it shows clear signs of violence we probably will never know the how or why.
