Considering formula in the evening - any implications?

[Moorhen]

DS is 14 weeks and used to sleep from 8-10, have feed, sleep 11-4/5, feed, fall asleep till 8.

Now, since his first cold and clocks going back, he's waking up for the 4/5am feed and refusing to go back to sleep. He's not a good sleeper during the day (catnaps mainly, 45 mins tops) and wants to sleep from 7pm, although I do try to keep him awake a bit longer.

I am desperate. DH spent an hour shush-patting this morning from 5.30am and the little blighter only slept another 45 mins. Am permanently tearful because I now can't get back to sleep if he does go.

Am considering giving a formula bottle in the evening (he already has a big bf and a bottle of ebm) just to see if it tides him over that one extra hour. I don't LIKE the idea, but as I say I'm desperate.

I would continue expressing, so it wouldn't affect supply hopefully, but is there anything else I should consider before I take this step?

oh, and I don't buy the formula-makes-them-sleep-longer theory since (1) he's been formula fed for a long time now, and just started sleeping through the night without a feed at about 9.5 months and (2)when I switched from bf'ing to formula, which i did for reasons other than his sleeping, it didn't affect his sleep at all.